Just being pedantic, but it requires souls - souls that could be used for other things. Again, just saying.
Souls are always farmable but in a PvP build, stats are limited (assuming you want to stay in an active PvP level range).
But yeah, in a first playthrough, pumping too many souls into your flame will hurt you elsewhere. Like when you get to Quelaag and she laughs at your fireballs. It's just...
... it's a huge shame that you can do pretty much the whole game with two weapons that don't require stats for damage at all (Drake Sword --> Lightning [spear/whatever you want]). Did the Dragon +5 weapons last until the end of the game in Demon's? I never used one.