Because it comes at the cost of not gearing the work toward the actual intended end format, which is uncompressed 16-bit 44kHz.
Present format mp3s will always sound bad, that is one undeniable truth. There is no point to be mixing specifically for a format that inherently professes a disdain for audio fidelity. Or at least considers it a distant second to the convenience of storage space.
Assuming that the music will end up in mp3 is fine, but that's a byproduct - not the actual product. Most of the time mp3 is used as the standard streaming format for really low-fidelity players like Myspace, Youtube etc. etc. but the end product still ends up primarily pressed and distributed via CD. At least my work does. So it really would come down to us mixing specifically for the people who would pirate the record online. Why would I possibly want to reward them over the people who care enough to actually purchase a CD and listen to the record as intended?
Sure, a lot of bands now distribute solely through mp3, but we've had that discussion already. Some of us want no part in it whatsoever. Hence taking us back to: 'What an awful concept.'