You need the extra AP and sustain in lane.
how about buying a doran's ring?
you lose 5 damage on hit, but you get 10 more ap which benefits your spells, passive and heals.
you lose 5g on hit but dorans costs 100g less. that's 20 hits to make those 100g. 20 hits is not a lot but with dorans you get the extra 100g right away with which you can buy a pink ward or a couple potions or whatever. i don't know how much money you usually make with t2 spellthief, but you keep t1, so you still have the gp10 and the basic on hit moneys
and 25% mana regen is roughly less or equal to 3 mp5 for most of the early game. a few minions secured with sona gives you a bit more sustain
worst part i think is wasting an item slot
what do you think?
...
also i just realized doran's rings are still using flat mp5. rito was pretty inconsistent with that shit, doran's, blue elixir and blue buff still give flat amounts. i'm guessing they don't want these items to scale? it's weird...