I will give some arguments:
WavPack outperforms FLAC in some pratical and technical aspects:
Better compression in some modes.
Don’t need to re-write the file when adding album art or modyfing tags, as there is no padding to run out.)
Supports 32bit WAV (although I wouldn’t know what use this would be for…)
Support wildcards in Windows command line (try that with FLAC)
Simpler command line options, compared to FLAC.
User base growing each day…
No 2GB file size limit inside Windows, FLAC Win32 can’t create a 2GB file in Windows, due to Windows issue (not the FAT32 issue here) - happens even on NTFS. This affects those who want to encode a multi-channel DVD Audio, for example.
WavPack has a “lossy” mode, as good as MP3, Vorbis and AAC.
WavPack use APEv2 tags as default tagging system.
WavPack, as FLAC, has also a ReplayGain tool WvGain.
WavPack, as FLAC, is supported in Windows & Linux and others.
RockBox Fuze Port has WavPack support, reason why Sansa should implement, so users don’t need to fiddle with firmware replacement, but have something official straight from the company.
RockBox people understood that the default recording mode should be with WavPack and not FLAC (for whatever technical reasons they chose it).
As a potential buyer, WavPack support would be definitely the break-through and could help sales.
WavPack has a BSD license.