VileR notified me about this thread, as I've been working on "perfect" composite color captures from real (hardware) CGA for a few months, and I thought I'd chime in on the 4:3 vs. 16:9 pillarboxing conversation. My stance: I deliberately pillarbox the footage I upload and distribute because I consider it future-proofing. Here is my reasoning:
When YT was trying to figure out 60p, they went through many stages of support for 60p uploads; there was a time when 1440x1080 (a perfectly valid HDTV, broadcast, MPEG-2 transport stream resolution) was accepted and interpreted as 60p, and then it wasn't, and then it was considered anamorphic 16:9 (which is valid for AVCHD footage), and so on. I don't know what they do with 1440x1080 these days but it was enough for me to realize they don't always know what they're doing. The containers I use (1280x720, 1920x1080) are 16:9 with square pixels and 59.94 frames per second -- these are HD-spec (and in the case of 720p, broadcast-spec) containers that cannot be misinterpreted as to whether or not they are 16:9/60p or something else.
Supplying a pillarboxed video also makes it very clear what the aspect ratio is supposed to be, because it is baked into the presentation. I always cringe when I see 4:3 videos stretched out to 16:9. Providing a pillarboxed video will drop into any professional or amateur video editing software, or player software, and play with the correct aspect ratio every time. In other words, I'm protecting the end user from themselves (and crappy software).
Besides, if anyone wants to use my videos in their own work, it is trivial to crop out the pillarboxing in any video editing software.