(Too lazy to read entire thread, so apologies if this was already said.)
I think the primary problem with summon spells has little to do with CT and a lot to do with bookkeeping.
First, summoned creatures often have the fiendish or celestial template applied to them. That means you've got to look up Eagle in the MM appendix, then look up the Celestial template, then apply it. And that's before you get to move the creature or make it attack something.
The bookkeeping gets even worse when you add things like Augment Summoning, Animal Growth, or whatever buff spells the party has in place or chooses to cast while the summonees are present.
Second, adding additional combatants to an encounter inevitably makes it slower. This is a minor detail at low level but a major pain at high level, when the summonees have many abilities (often the reason you summoned them). This creates additional bookkeeping for the players, who have to keep track of the creature and what it can do, and for the DM, who has to decide if the monsters react to the summonee by attacking it, or ignore it and go after the PCs, or what.
Third, the proliferation of monsters that have an entry like "This creature can be summoned with a summon monster N spell" just exacerbate the bookkeeping problems. Now the player has to keep a list a non-Monster Manual creatures he can summon. And then he needs either another book or a photocopy of the book's pages. This is similar to how wild shape and shapechange get out of hand with more and more monster books being published.
I like the idea of summoning, but frankly it's too much of a hassle to use (as a PC) or to allow (as a DM).