That scheme is horrible. ? should never be part of a rest-like url and you certainly shouldn't request more than one id at a time -- the data you should show should be included in the JSON string.
Why would you say something like that? There's no basis for that at all. From a REST POV, URIs are just opaque identifiers, the characters they're made up from don't matter a bit.
I always interpreted /collection?xxx=yyy as adding constraints to a collection - that's perfectly "REST-like" (actually URIs don't really matter, but I suppose you're talking about conventions). And in that respect /collection?ids=1,2,3,4 is indeed just a subset of /collection
For a single resource you can still use /collection/1
