AIFF do support tags, many other id3 editors use it.
As many serious digital music listeners prefer to use uncompressed formats such as AIFF rather than FLAC, it would be extremely helpful if Picard would support it. After all, it is the best tagger.

Just a note, this is very unlikely to happen unless somebody adds complete RIFF support to mutagen, which is a rather large task.

Are there any other reasons besides this to encourage someone to do the RIFF support?
Or is there any other tagable uncompressed audio format that one could use?

Why does it have to be uncompressed? FLAC is lossless, and is used by most "serious digital music listeners" that I know?

voice : If you want to do some mixing on many tracks this will ease the CPU by not having to decompress many stuff at every second.
Of course, on the other hand, your HDD has to be fast for uncompressed mixing but this part is usually OK.

No worry, just found out that kid3 does most of the stuff I need for aiff anyway.
So I'll skip Picard for the moment.

Sure, but you still didn't answer the question as to why FLAC is unsuitable for your needs. (patate12 gave one possible answer, albeit not that strong in my opinion given the relative performance of CPU/memory for decompression these days)

To be clear, I'm not saying that such arguments don't exist (for one, that many pro-audio software and hardware may still use it is a decent one), but it's not clear that this is your use case.

I opened a ticket against Mutagen: https://code.google.com/p/mutagen/issues/detail?id=146