I don't know how they pan out, but b-sides collections often work well on their own as stand-alone releases. Not "proper" albums, per se; rather, they're enjoyable collections of orphaned songs. Listening to Belle & Sebastian's Push Barman to Open Old Wounds over the weekend was a great reminder of this.
For a lot of bands, b-sides are quickly written, quickly recorded and tacked onto a single to help sell it. In all fairness, b-sides often give bands a little more room to stretch out with songs that normally wouldn't fit their mold or style. Sometimes these songs are relegated to b-side status because there were stronger tracks and they would disrupt the flow of the album. Then there are bands that have so many songs written and recorded that they don't know what to do with them all. Because of these factors, an alternate album (or albums) is created.
Case in point, the singles from Idlewild's previous album, The Remote Part, featured a number of original songs, a few choice covers and a couple of live tracks as b-sides. All of the tracks are strong (especially, the originals, "All This Information" and "The Nothing I Know") and when you arrange the songs together onto one disc, you have a pretty solid album. The b-sides are not as strong as the songs that ended up on The Remote Part, but they shouldn't be thought of as forgettable afterthoughts.
Some artists take the easy way out with b-sides by using remixes and live tracks instead of new tracks. Some remixes are interesting (see the bonus remix disc on the Doves' b-sides record, Lost Sides) but they are oftentimes a test of a listener's patience. Live tracks tend to be note-for-note rehashing and not the best performance of a previously-released studio track.
One of my favorite b-sides collection is Sugar's Besides (still love that title). Taking non-album songs, alternate versions of previously-released songs along with some smokin' live tracks (especially their version of the Who's "Armenia City in the Sky"), this may be my favorite overall Sugar record. Their proper albums were a little short and very safe. At 17 tracks, there is plenty to marvel at and enjoy.
In the example of Belle & Sebastian, they released a number of non-album singles, complete with non-album b-sides, while they were on Jeepster/Matador Records. Plenty of worthwhile tunes were found on the a-sides and b-sides, like "Legal Man," "Belle and Sebastian," and "Take Your Carriage Clock And Shove It," but they wouldn't fit very well on a proper album. Combining them onto a 2-CD set as Push Barman to Open Old Wounds, you realize how strong this band is even when they're doing non-album material.
I have friends (especially Mark) that collect every single a band/artist releases in order to have everything. For me, I often wait to find a b-side on an MP3 blog or on a reissue. I have yet to go through all the bonus tracks on the Elvis Costello reissues on Rhino. I'm still trying to dig deep on the album tracks.
People speculate that the use of b-sides on a single will eventually fade away as downloading singles is now the way to buy singles. Well, as long as there are compilations, bonus tracks are needed on foreign versions of albums, there will be no limits for us curious completists.