I practice quite a bit on my 8-string bass: triads, scales, etc
As well as running through whatever my current set list is and learning new songs.
I find that it has strengthened my fretting hand quite a bit, as well as forced me to
be much more precise when picking (both fingers and with a pick) as as not to get string rattle and bad notes.
When I then switch to my regular 4-string bass for gigging, I feel like I am a monster. Strong, fast, accurate and can play it for much longer.
-New York Bassists Club #68
*Rockin' Carlo Robelli basses since 2006!*