Originally Posted by Cold Fusion
The summoners are definitely bound to their champions in some ways (they feel pain when their champion does, see season one trailer).
I think you could say the reason behind the increased respawn time as the champion gets more powerful is that they're harder to control as they level more and more.. Which would also maybe explain why they can't pass level 18 (summoners can't control the champion after that?).
Idk, just a big guess.
The summoners feel pain because their minds are directly linked to one another. As is pointed out very poignently in every League Judgement, the inductee must open their mind to being taken control of, or at least guided in some fashion. It's always expemplified with a single line in every Judgement: How does it feel, exposing your mind?
I think this accurately portrays the idea that both summoner and champion are completely linked, knowing every thought from past and present if they so chose to delve into the recesses of their minds. Pain is also a state of mind, by some beleifs, so it stands to reason that when a champion experiences pain, the summoner feels the backlash of the total mind synchronization they have going on.
As for summoner spawn time? The more powerful you are, the harder it is to rebuild you the same. Simple as that. =)