I think both of you are misunderstanding the Tiers system. You're looking at the classes, like Sorcerer, and then comparing how the Summoner stacks up against it.
The relevant question is what game-changers are available to the Summoner? I have no idea, I haven't perused the list. But, that's where it will matter. For example, 6th level spells, but from a very restricted list, could leave you at Tier 3. Very limited casting, but free to cherry pick the most game-breaking spells available (think of something like the Divine Crusader) could be T2 or even T1.
As LordBlades notes, characters who are great at combat, say the Uber-Charger, who wrecks combat, tend to be T3.
As I understand them, and I am far from expert on this, the Tiers represent, essentially breadth of capabilities. Along with maybe a dash of "how much is this going to fuck up my campaign?" Tier 1 characters are Tier 1 b/c they can do everything in a game -- kick ass, take names, make friends, influence people, build houses -- and also have access to a variety of abilities that can potentially make things go pear-shaped. I don't think the Tiers are the be-all, end-all of game mechanics heuristics, but that's what the thread is about.
EDIT: the above was written before Madwand's post immediately above, so take it with a grain of salt. That being said, wouldn't a Rogue, or similar skilled character, who also happened to be good at combat, be T2 by that logic? It just seems like you could say the same thing about the Binder or the Beguiler, and they both sit in T3 land, according to JaronK. But, ymmv.