I am in favor of getting good FLAC support in MC. Whether that comes in the form of native support or a quality plugin makes no difference to me - as long as I can import/encode/decode/transcode and *tag* effectively, then I'll be happy.
My reason for wanting FLAC support is that currently, Slimserver has full FLAC support so that I am able to transcode FLAC -> MP3 for streaming purposes (works with any stream capable player).
As far as portable battery life, I don't see how that would be an issue at all. FLAC uses hardly any resources to decode. I am not certain, but I suspect that a high quality MP3 would require more computation power to decode than FLAC. The only drawback on a portable is file size.
If, in the future, someone writes a module for Slimserver that will allow me to use APE (there have been attempts, but nothing solid yet as far as I know), then I won't have a use for FLAC at all. I don't have a portable player, and I doubt I ever will...
-Alternatively- If, in the future, MC supports streaming to non-MC players, then I will have no need for FLAC.