I've never bought fonts because of either online testing or trial licenses. Neither has a cut-down version nor a free style been an inducement. I purchase fonts either because the job dictates it or because I like the shape and glyph coverage I desire or need is present.
How a site looks is also not a consideration to me as long as I can see what I am buying. How well a site works would be an issue to me if the font is available elsewhere.
I only rent ID when I need to—and am not doing so right now—but do have CS6. AI seems to work just fine here. Fully, at least with my font.
However, I think it is Minion/Myriad PRO fonts (versus the Std versions) that have some other features and I don't think it will fake them. Of those two, I think Minion Pro is the most complete, feature-wise. Might be wrong. I really never use them.
As a person who uses type more than creating it, I use the heck out of stylistic sets, alts, etc. Every designer or layout person I know expects such features.
To me, it is more of a why create both italics and obliques. What would a person purchasing a family expect? Would they expect a stylistic set for the obliques versus a family member? In this case, I think I would expect a family member.
That said, in my current font, a historical repro, there were examples where both slanted and a weird mix of true italics were used in print. And so I did put the obliques into the regular/semi/bold styles as a stylistic set.