The only downside to keeping everything in a lossless format is that over the years new formats emerge. mp3 used to be the only game in town, but now we have multitudes of lossy formats to pick from. By having your collection in mp3 format, you aren't able to say "hey, this new format looks cool, let me switch to that". By storing everything in a lossless format (FLAC), you can convert for mobile as you see fit.