"A very ironic symbol for him to be using, since the last thing he stands for is purity and beauty, but I love how he still uses it."
I don't really agree with this. I rather think that he's got a bleak vision of humanity, and, even if he seldom mentions it, that he longs for genuine "purity", or "innocence", or whatever you may want to call it. Just look at ANY reference to his beloved daughter. Also, in "Ham on Rye" he depicts a cat that was thrown to a dog as "pure and white", and pities him as an innocent victim of human brutality.
As for the symbol of the rose, Bukowski does use lots of references to flowers, but they don't just stand for beauty. They seem much more ambiguous. In the collection "Burning in Water, Downing in Flame" (the one I know best so far) they are also deadly and there are almost cannibalistic undertones. In "the Flower Lover" the flower cuts the poet in pieces, and is still seen as beautiful. In "hooray say the roses", the latter are clearly related to blood and violence- I'll just quote the first lines:
hooray say the roses, today is blamesday
and we are red as blood.
hooray say the roses, today is Wednesday
and we bloom where soldiers fell,
and lovers too,
and the snake ate the word.
I used to think it was because roses have thorns, but that sounds too easy an explanation. In any case, flowers are indeed interesting symbols in the context of Buk's poetry.