I said in there that the benefits of it are +1 and reach. But besides, one AO isn't a big deal. And besides, if AO's were such a huge deal everyone would be using a spiked chain anyways.
Will you still get the -X penalty if your weapon resizes? You're not trained for it. So you might end up with another penalty to hit anyway.
Don't get me wrong, reach is very nice and will provide lots of extra AO's. However it'll probably be limited in duration so the Wizard will need to touch the fighter, that usually is enough to discourage it in a lot of situations. IE, fighter races off to engage in melee, wizard stays back to protect own butt. If Enlarge/Bulls Str, etc are 1min/level they'll probably be cast during battle in most situations. So the Wizard will need to be close to the fighter who's trying to engage in melee. I see it as being useful, but not a must-have or anything.
In my experience as a fighter/wizard/DM I'll probably end up using the new Haste spell much more, it affects multiple people which is freaking awsome. A wizard casting enlarge at later levels will be rare, but fighters with potions of enlarge should be nice to see. Especially if you know you'll be fighting giants soon. But the -2AC is significant