BFT is strong, but for a few more reasons than just the passive. On top of its passive, it's a giant stat stick giving AP, HP, CDR, and Mpen for a fairly cheap price. For 2950 it gives casters everything they want damage-wise. It's a bargain compared to something like Deathfire Grasp which costs 3000 and only gives AP, CDR, and an Active

Let's Compare the 2 shall we? Let's do some math crafting here. First, it's 3.5% over 2 seconds now, so thats 1.75% every second/charge on BFT. Lets say you blow all of your charges at once on a single target dealing full damage, thats 31.5% of their max HP(realistically this takes 18 seconds to achieve but we are simplifying things). DFG only gives you 15% but has the added bonus of the damage increase(which is hard to weigh). DFG has a cooldown of 60 seconds. It takes 27 seconds for BFT to fully charge itself.

Now, DFG does have 30 more AP and 5% more CDR, while BFT grants 15 Mpen and 250 HP which DFG has none of, yet DFG costs 50 more gold. SO, if you were to ask me, which you did by posting in a public forum , BFT is just too cost efficient for what it does and needs a stat change, not just a tweak to its passive.