Darchias's page

No actual rule preventing it, just plain old logistics.
It costs a full round to use the spell, and the critter you are summoning doesn't show up until the beginning of your next turn anyway. Hopefully you have a way to prevent your foes from getting to you, because if I'm a monster with more then 10 intelligence, I'm prolly gonna figure out that the gestures that funny lookin' guy is making is why these, say, fiendish wolverines are showing up in my business.
In addition, spending more then one or two summonings on a combat probably won't be necessary in the lower levels. Higher level fights might require it, but anything lower then a, say, CR 7 encounter probably won't need more then that if the rest of your party is backing you up.

An example of a spell requiring line of sight but not line of effect is summoning. You can designate any place you can see within the range of the spell for the creature to appear, so long as the enviroment can support it. Keep in mind this means any place you can see with your physical eyes, so you can summon through a window, but not utilizing a mirror.