It depends on two things: league format and your roster.
One of my leagues is a H2H league that counts IP, W, and K, all stat categories that are enhanced by making extra starts. If I'm in a tight race late in the week, occasionally I will pick up a throwaway in a good matchup to try to put me over the top in those categories. An obvious drawback to this is that the throwaway pitcher could come out very horribly and ruin a lead in WHIP/OBA, etc.
The second thing to think about is the quality of your existing rotation. In that same H2H league last year I had a pretty weak rotation, and thus I was constantly seeking help off the WW. But this year my SPs are very strong, and as such, I'm not making those spot starts.
I hope that is helpful. I've played with and without them. One strategy might be that if you have an open spot you're not sure what to do with, make a few spot starts, and if you find someone who is consistently performing, consider taking him on as a permant add (i.e. look for this year's Cliff Lee).
"This guy leads the league in most offensive categories, including nose hair..."