If you're going to add spell casting time to initiative, then you should add weapon speed factors to initiative, and armor check penalties to initiative, and have multiple attacks occur later in the initiative order. Just to be fair.
I'm not trying to be fair - I was curious if anyone had tried this - the intent of this type of rule would be a (very slight) nerf to casters - which would allow extra reaction time to try and disrupt a spell being cast.
In many (most) cases of combat I've run I expect this wouldn't really change much - however it would open up new tactics and make casting slightly more dangerous.
A side effect of this would be limiting a casters movement on the field (to move before cast) - which would also change tactics.
Casters still alter reality and in general have 'combat ends' options - but it would open the door to more chances at disruption - and make higher level spells be more dangerous to cast. There is a definite meta-game change that would take place and it would shift some balance of power to melee (more so the higher level everyone is - which is where the balance swings badly towards casters).
Or are you suggesting melee are balanced against casters level 10+ ?