Sure it's justified.
But i can't see why you would want to?
CD's are higher quality than MP3's, that's not secret. The problem with a lot of MP3's from the internet, however, is that they are ripped by people who have little knowledge about MP3 codecs, general music quality and what importance Joint Stereo makes. This means that they often use terrible MP3-codecs, use normal Stereo (which quality-wise is around 40%+ inferior to a proper Joint Stereo implementation) and assume everything is "High Quality" just because they set the bitrate to 320.
Unless you have an MP3-player that doesn't support the AAC format (most do these days, including iPods since AAC is the format iTunes Store sells music in), my recommendation is that you rip your CD's yourself again to AAC, since it's a very superior compression format to MP3 and always uses Joint (Mid-Side) Stereo. Sure it takes time, but at the end i believe it's worth it.
If you happen to use iTunes for ripping, use the settings in the picture below, except the Bitrate, which you can set to whatever you like depending on your disk space availability/quality requirements. From personal blind listening tests, i can tell you that i can't make out the difference between 160 kbps iTunes AAC and CD-quality, but all audio codecs have so-called "Killer Samples", meaning that there are some music pieces that will have compression artifacts with a specific codec if it's set to low quality, which is why it's good to use a bit higher quality than normal, even if you can't hear the difference at lower quality normally. I personally recommend at least 192 kbps (which is around the equivalent of 230-240 kbps LAME MP3, LAME being the best MP3 coder on the market). Whatever you do, however, don't rip to MP3's using iTunes, their MP3 codec isn't very good. 192-320 kbps VBR AAC is close to the best (lossy) compression you can get, if not the best.
If you have any further questions about proper ripping, then feel free to PM me and I'll be happy to help you out
