Of course, the single disc idea is for when we're talking about a feature that's at least 45 minutes to an hour long. When babyhum put out Heroes/Villains, they were both combined on one disc. Firstly, they are both short, so quality doesn't suffer when they're put on one disc; and secondly, they really just belong together. Another example of that is the Gonzo "Making Of" discs: two features per disc. That works well, not only because they're all very similar, but also because the first two were released together at one point anyway, and the second two are mostly Jedi so they make a great pair.
Obviously when dealing with commercials, trailers, and short things like that, they will need to be grouped together or on a disc with a main feature. I like them to be grouped in a way that makes sense, either chronologically (they were aired around the same time as the feature), or by subject matter (they somehow relate to the feature, or are similar in some way). Putting a copy of the 1978 Donnie & Marie Star Wars special on a disc with an Episode III trailer and a featurette about the 1997 Special Editions, say, would just not fit. Putting it alongside the Bob Hope Star Wars special and the Richard Pryor Star Wars Bar segment definitely would.
I also like to keep my OT and Prequel-Era material separate. Perhaps it helps me sometimes pretend they're from alternate universes. Even the Special Edition OT stuff is kind of a grey area, and I tend to lump it in with Prequel-Era more often than not. Rule of thumb for me: if it's 70s-80s it goes in one place, and if it's 90s-00s it goes in another.
So I guess my vote is: I'd rather have a not-full DVD with like material on it in high quality than a DVD crammed with stuff that's dissimilar, lower-quality that what's available, or both.
--SKot