Dont forget that the incantation mana drain gets severely gimped vs mana prism.
16 mana (IIRC) to roll 8 dice. the amount rolled gets transfered.
This spell rolls an average of 8 mana drained. which makes this spell break even for the caster. (he end up paying 8 net, but opponent loses 8 as well. (this is average))
With mana prism the wizard end up paying 16 without any refunds (ever), and opponent lose average of 8.
And thats 8 mana that gets returned eventually.
Also note that if you happen to get rid of the mana drainers (mana siphon, upkeep enchantment, mana leeches, supression orb etc etc) while keeping your prism up, then you will eventually empty your prism from mana resulting in a total of 0 mana lost where as the enemy controller spend tons of mana trying to control your mana.
I can see this prism fit well into decks that have a great weakness vs mana denial wizards.