On the Importance of the Wine-God
Dionysos-Bacchus may not have been the most powerful, most important deity of Greece or Rome, nevertheless, for cultures to whom feasts, agriculture and procreation were of no small significance, such as were those of the Greeks and the Romans (and would still be the case today), the god was certainly integral to everyday life. Wine is actually a nicely concise shorthand or connector for all these aspects of life, because its growth requires fertility, which affects cultivation efforts in general; parties and other such communal gatherings in numerous places the world over, for millennia, have generally been thought to be drab or uncool sans alcohol; and there have always been strong connections between the fertility of the earth and the activities of reproduction for both human beings and animals. Hence the realm presided over by Dionysos-Bacchus absorbs all these aspects of daily and communal life into itself.
The reason he stands somewhat aloof from the rest of the pantheon(s) is because he represents the removal of inhibition which many of the more conscientious, civic-bound deities represent. Where they are urban, proper and self-contained, he is wild, alien and unrestricted. Where they would enjoin clear thinking, he is the advocate for the distorted state of mind required in order to escape the mundane drudgery of everyday existence.
An Imported Rendition of Himself
Notwithstanding those facts, I would certainly agree that, as understood in the mainstream Olympian and Roman religions, the god of wine and drunkenness is certainly a glaring contradiction for an ascetic, vegetarian lifestyle as envisioned in Orphism: the way of the mystic Orpheus, who, most ironically, in one version of his death story, was violently dismembered by revelling followers of Dionysos who were upset with him for refusing to participate in their orgy because he was in grief mourning the loss of his wife!
Certain ancient Greek writers do explicitly distinguish the mainstream Dionysos from the one venerated in the cult of Orpheus, saying precisely what you're asking about: that the Orphic Dionysos is a foreign deity who was imported—stolen, even—from Africa or the Near East and transposed onto the Greek (and thus later Roman) wine-god. Orpheus is supposed to have copied the mysteries of the Egyptian Osiris and brought them into Greece in a different form as the mysteries of Dionysos.
Herodotus writes that the man who did this was the great prophet Melampus, a descendant of Aigyptos [Aegyptus], the king from whom Egypt got its name. In Book 2 of his Historiai, "Histories," Herodotus recalls the journey made by Aigyptos' cousin, the Tyro-Sidonian prince Kadmos [Cadmus], in search of his sister Europa, after whom Europe was named; and how Melampus learned the Canaanite ways of Dionysos mostly from Kadmos and the Canaanites with he whom he came into Boiotia [Boeotia]. (According to the Egyptian writer Nonnus, these Canaanites were originally Egyptians who had migrated to Canaan from Memphis with Kadmos' father Agenor. Kadmos and Europa are supposed to have been born and spent their earliest years in Egypt, in the house of their uncle Belos [Belus], who was king of the land before his son Aigyptos.)
Now for anyone who knows the basics of Dionysos' family tree, there's a big chronological discrepancy in Herodotus' claim about Melampus learning Dionysos' mysteries from a young Kadmos, because Kadmos is the maternal grandfather of Dionysos, and the wine-god was born a good long while after Kadmos had left Canaan and become king of Boiotian Thebes! It could therefore only mean that either Herodotus rejects the traditional family tree of Dionysos, or he's outright saying that the Dionysos of Melampus is quite distinct from the Theban grandson of Kadmos.
Zagreus, the First Dionysos
The need to distinguish between these Dionysoi would appear to be the inspiration for the myth of a Dionysos who was before Dionysos, that is to say an incarnation of the god before he was born as Kadmos' grandson. The most detailed rendition of just such a thing is recounted by Nonnus in his epic poem the Dionysiaka, in which Dionysos is a reincarnation of an ancient, beautiful and monstrous shape-shifting baby god named Zagreus. Like Dionysos after him he too is a son of Zeus, sired upon Zeus' own daughter Persephone before she becomes the wife of her uncle Hades.
Zagreus, born before the existence of humans, was Zeus most favourite child, of whom he was so fond that he planned to abdicate his throne in favour of this child, whom he even allowed to play with his lightning bolts. Horrified at this state of affairs, Zeus' wife Hera induced some of the Titans to lure the baby away, and they tore him to pieces which they devoured. Aggrieved at this Zeus used his lightning to reduce the murderers to ashes, from which, after being mixed with earth and water, humankind then arose. Zagreus' heart was somehow retrieved and continued beating. Centuries later Zeus then implanted this heart into his lover, Kadmos' daughter Semele, from which union Dionysos was born.
The Orphics used this version of the creation story to explain why human beings are stuck in a wheel of death and rebirth (virtually identical to the concept of saṃsāra in Hinduism and Buddhism) with the human soul (Dionysos Zagreus) and body (the Titans) as separate entities, the former trapped inside, or being devoured by, the latter.
The New Encyclopædia Britannica, Micropædia, explains this understanding of humans as the combination of "an earthly (Titanic) nature that had to be suppressed and a heavenly (Dionysiac) nature that had to be cultivated. This was accomplished by living an Orphic life, which included abstention from meat, wine and sexual intercourse.” (With all that being so then the Dionysos who reincarnated Zagreus is definitely not an Orphic.)
Some More On Importance
The oldest extant written reference to Zagreus, from a fragment of the now-lost epic Alkmaionis, says that he and Gaia are "above all other gods". Herodotus believed that the Arabians of his time worshipped only Aphrodite Ourania [Urania] and Dionysos, the latter of whom he says they addressed by the name Orotalt. Brewer's Dictionary of Phrase and Fable attempts to explain the purported foreign name as a corruption of the ancient Arabic term for "God Most High."