I guess I'm in the minority, b/c I think there is a place for spot starting in Roto before the end of the season.
Ordinarily you'd like to have a full staff of healthy players, but that's just not a reasonable expectation...players get hurt and some for long stretches. Bedard, Schilling, Harden, Thomson, Sheets, Wood, Prior are just some of the notable pitchers who've seen the letters "DL" next to their name this year.
Also, b/c most people are trying to reach their IP max at the end of the season so there is usually a scramble for spot starters. Why not pick them up mid-season while they still have something in their arm and you have first crack at them?
So to answer your question about what I look for...
1. Playing a weaker team (ex. TB, COL, KC, HOU, etc).
2. Ordinarily I like them to be @ home. I've found there are just some pitchers who aren't road tested (look at the splits on Rodrigo Lopez, Brandon Backe) and are more predictable in familar surroundings.
3. I check to see if the opposing has had any success against them in the past (i.e. Batter vs. Pitcher stats). Sure Miggy has hit them in the past, but how has Biggie or Sosa done?
4. Lastly I see what they've down in their last 3 or so starts. For instance I think in most instances I would pick Greinke up for his start against the Dodgers, but he's been hit hard in his last few starts.
I'm sure I'm forgeting something, but I hope this enables you to make your own decisions.