It's something I've always done. I not only enjoy the challenge of it but it also allows you the luxury of only drafting 3 maybe 4 fulltime SP in standard leagues and concentrating more on hitters.
In the Cafe Challenge League in which we have more than the usual number of bench spots, I lost Myers early in the season which gave me just 3 fulltime SP's. Haren, Hudson and Halladay. With Halladay on the DL I've spent the past month or so spot starting almost daily.
There are decent options in all standard leagues to use and you can often unearth a hidden gem that ends up on your roster permanently. With good management, your spot starters can give u decent 3rd - 4th SP numbers.
I've made 15 spots starts to date, 12 of which have been quality starts but only have 5 wins to show for it. ERA 3.9 Whip 1.18 K's 64 in 90.2 IP. They are better than 3rd SP numbers.
When you're right no one remembers, when you're wrong no one forgets - NZF