There's nothing "wrong" with it per se - it just doesn't work the same as any other hard disk in the DAPS that Rockbox supports.
If you search on the wiki you'll find an absolete page that says why it took so long for Rockbox to support it in a reasonable amount of detail.
The only downside to using the 80GB for Rockbox concerns when Rockbox itself writes to the disk - which as pointed out above, only happens when you save a playlist, or a config file. And even then, the "downside" is that it takes four times as long as it does with any other Hard disk based DAP - and you probably STILL won't notice that, since playlists and config files tend to rather small.
Perhaps if you create a playlist with every song on your DAP in it it'd be noticeable - but even so - it doesn't bother me much. I don't do it enough for the performance hit to matter to me.