That's true, unless the original audio is 24-bit and your audio driver is sending 16-bit audio to your sound card, or similar sorts of loss are happening post-decompression.

Many devices have hardware acceleration for mp3 decoding, which also helps with battery life. I'm not sure how many devices have hardware accelerators that can be used/repurposed for Vorbis decoding in the Spotify case. I believe it's less common for hardware decoders to be useful for FLAC decoding, so there may be a battery life penalty for re-encoding mp3 or Vorbis audio as FLAC. (I also presume everyone realizes re-encoding lossily-compressed audio using a lossless codec likely pays a size penalty without any quality increase.)