Dude, those files do not become the same as the MP3s once converted. I'll really quickly describe how the formats differ: all the ones EJ listed contain COMMANDS and sometimes SAMPLES which a sequencer interprets to "arrange" the music. MP3s are frames of CODED SAMPLES (redundancy is removed) and contain no commands: it is only a linear stream of samples, like a WAV file is, but with the difference of being coded (technically, PCM data is also coded data, but...)
If you convert your SPC file to MP3, all you do is let the sequencer output the samples that are arranged from the SPC to create PCM data that is then passed through an encoder which (tries to) remove the redundancy from that. You basically get "less" data left from what the sequencer output.
The formats above are highly optimized methods of representing the music, and when played back and converted to MP3, don't become higher quality or anything. MP3s of videogame music are usually taken from CDs of the OSVs from the game, a completely different source. Usually these are better quality than the arranged ones.
The arranged music is like the "source code". The actual streaming PCM data is the "binary". You can create a binary from the source code, but not vice versa (or at least not without a ton of work). The original "source code" from which the music on the CD is created is usually not the same as that which is used in the above formats. Usually the sample precision is less, or certain effects have been removed (because there are less voices available, for example).
If you want to know why the music on the CDs is better quality than the music in the above formats, I advise you to read up on this stuff.