My liking them is the criteria for a purchase. That applies, I imagine, to every sale ever for all consumers.
As for redundancy, that is true only if a writer has a very limited imagination. The types of stories that can be told with an Ultimates or a Defenders book can be different than what we see with GotG, Avengers, X-Men, FF, etc. The point of more teams, of course, is to expand opportunities for characters who are just wasting away in limbo. Characters that you won't see regularly featured in the other books.
Depends on the character. Invisible Girl and Things power up stuck (she used to just be invisible, now she's force field queen, for instance, and Thing is much stronger). Marvel Girl and Cyclops power ups have stuck (his eyebeams used to run out of power, she used to be able to telekinetically lift as much as she could physically lift). Angel's radical power up to Archangel has stuck. Luke Cage used to be half as strong as Tigra, and now he's one of the heavy hitters. Nova and Captain Marvel used to be flying tanks, now they have cosmic-tier blasty powers and those powerups don't seem to be going anywhere.
Spider-Man occupies a specific niche, and powering him up kind of takes him out of that, so he's less likely to get a lasting upgrade.
I don't know, the thing about the Defenders and Ultimates is that people like the idea of the teams more than the teams themselves. They are the superhero comic equivalent of buying a homemade ice cream maker. Everyone is always excited at first...and then everyone forgets about it. As for the characters wasting away in limbo? Well maybe there is a reason why most of them are there. The thing about the Avengers is that the roster is always changing and most of the heavy lifting is done by the characters who don't have books.
"The Marvel EIC Chair has a certain curse that goes along with it: it tends to drive people insane, and ultimately, out of the business altogether. It is the notorious last stop for many staffers, as once you've sat in The Big Chair, your pariah status is usually locked in." Christopher Priest
Lol, it won't, Spidey ain't the only one Marvel doesn't want to age.
I had repressed that, great...
In your opinion, sure, but that's irrelevant because Spidey ain't someone to whine about things like that, specially if he talks like he's in a fanfiction written by a 12 year old talking about a Mary Sue.
It's annoying he got a downgrade if anything, he really doesn't look as strong as he used to be in the 80's.
Fights themselves don't help too, characters just pose at each other, which hurts him more 'cause he barely uses his speed when fighting.
Reading List (Super behind but reading them nonetheless):
DC: Currently figuring that out
Marvel: Read above
Image: Killadelphia, Nightmare Blog
Other: The Antagonist, Something is Killing the Children, Avatar: TLAB
Manga: My Hero Academia, MHA: Vigilanties, Soul Eater: the Perfect Edition, Berserk, Hunter X Hunter, Witch Hat Atelier, Kaiju No. 8