MBAFF is a kind of interlace that AVC streams can have. Basically the image is separed into blocks, and each block could be either progressive or interlaced. So in the same stream you could have content telecined to 29.97, progressive at 29.97, interlaced at 59.94, and even combinations of two or more in the same frame, considering the block division. It is possible to find vfr material on DVD, however on DVDs you don't have different content separed into blocks (you can have telecined content with full field interlaced content on top, though, and that isn't nice

), so it's easier to deal with. OTOH, if you find content a section in a MBAFF stream that has both progressive and interlaced blocks in the same frame, you have to either do a lot of fancy stacking to keep the progressive footage as progressive and treat the interlaced/telecined content properly, or just nuke the whole thing, which could lead to a loss of quality in the progressive parts of the image. There probably exist some filters that can recognize progressive content and leave it alone while dealing with the interlaced content, but I'm not sure which/how good they are. IIRC I was told that some versions of tempgauss_mc can do that, but there are over 9000 versions of it around so it's kinda hard to know which do and which don't, if any version actually does...