I wish there were at least some kind of secondary A:TI title that could spotlight the teams for the other 45 or 46 states that aren't covered in major comics already. Maybe not weekly, since that might be too much of an overload if it's an ongoing series, but a secondary anthology title where different creators tell done-in-one stories about an Initiative team each month. That'd be cool. Heck, even a back-up in A:TI itself would be nice. I feel like we've barely gotten to know any of the Initiative teams besides the Avengers, and now it's looking like they're going to have even less of a presence since the status quo has de-emphasized the post-Civil War status quo a bit with Dark Reign.
You have a point there, the question, though, would have been if AVENGERS: THE INITIATIVE was selling well enough to have justified a spin off, since it itself was an Avengers spin off (some Avengers were in it, but the title really wasn't about them). Without crossovers to boost sales, the book averaged about 60k, which is fine but not always spin-off worthy. It usually is safe to assume a spin off of a title will sell, if one is lucky, half of what the parent title sold. Considering NEW AVENGERS often averages 90-100k, A:TI almost adheres to that rule. Of course, this sort of realism based number prediction usually isn't practiced by some editors, such as the ones who thought the below 35k selling GHOST RIDER title would support a side mini on Dan Ketch, which is selling terribly.
(Given that logic, the fact that THE LONERS, a spin off mini from the 25-27k selling RUNAWAYS series under Vaughan, sold about 12-15k as it ended really wasn't surprising to anyone but an unrealistic expectation.)
Plus, to be frank, anthology titles haven't done very well for a good, long while. MARVEL COMICS PRESENTS really only worked out for a bit by focusing the lead story on Wolverine during the peak of his popularity during the late 80's into the mid 90's. After that, anthologies have struggled. X-MEN UNLIMITED had two stabs within the Joe Q era that didn't work, and neither did SPIDER-MAN UNLIMITED or the latest MARVEL COMICS PRESENTS. Even AMAZING FANTASY, which helped launch a few new characters, petered out within about two years. They usually come down to "why do we care?" type situations. Anthologies rarely have the top draw talent, nor the importance of the current company storyline, to make people care. Is it fair that too many readers, even some of us, have a "if it isn't directly important, it is filler and not worth my time" attitude? No, but both Marvel and DC have trained their readers to look at books this way for at least the past several years, and then both are mind boggled when books that have the impression of "not counting", surprise, tumble down the Top 100. Which is a shame because quite a few good books do die that way. Considering most of the industry relies on the "hardcore" fan who buys at least 5-20 books a week, one can't blame many for expecting the books they do buy to "matter". Marvel hasn't been able to accomplish that for an anthology, beyond the occasional annual/special one shot, for years now.
While it is true we barely got to meet a lot of the new teams before they were "disassembled", some could claim that is always the perennial price of the "yearly event athon" at Marvel. You barely have a chance to get used to any status quo before it begins to be torn apart to make way for the next. Given all that, I think Dan Slott and later alongside Christos Gage (who came in after issue #7 in the first annual) did a fine enough job with the one title they had.
'Course, in an age when it seems ANY excuse is given to justify yet another Wolverine mini/ongoing/one-shot (and not all of THOSE sell well, as in above the Top 70, believe it or not), I can understand some folks wondering about branching off A:TI a year or so ago. I don't quite agree, but I understand the point.