Any software that is desirable on its own will be sought out on its own. Plain and simple. There is no need to piggyback adobe PDF reader in any software that doesn't use PDFs because people who need a PDF reader will seek one out.
Any sort of parasitism is trying to circumvent this process by convincing others that they need software that isn't directly needed for the problem at hand or otherwise forcing people to install software. And that's the type of behavior styled as "questionable" in this discussion. Tactics like omitting a "no thanks", while perfectly legal, seem as questionable in many other contexts.
I feel like you may be right for the wrong reasons. "Any software that is desirable on its own will be sought out on its own." is the prelude to many a failed software project. People say the same thing about films, games, apps, etc... and it's almost universally untrue.
If you are right, it's not because no good software aught to be promoted and thus this (and by extension of your logic most any type of promotion) can't be legit, but rather that there doesn't exist a non-scummy way of conducting this particular type of promotion. Is that true?
I'd say that piggybacking on installation process is not a valid way of promoting other software. If I go to the cinema to see movie X, I don't want to have to see half X, than Y, than other half of X, just because the distributor thought that Y ought to be promoted.
Sure, advertise the crap out of your product, but do it in contexts where people expect to see them. It's like I'd let a friend through my doorstep, and a random salesman tried to sneak in while doors were open.
And first make sure that whatever you're promoting has any value for user. I'm yet to see a single thing install-bundled that is net-positive for user.
I'm not saying that "no good software ought to be promoted", but rather that the promotion should happen at the places where people would seek them out (e.g. Google). Advertise for photoshop where designers would go to look for software (e.g. in design magazines, google, etc).