Plus stats are useful in single-season progs because you see at a glance who's above or below average in that particular season. Generally speaking, if you have a starting pitcher who's 100 or higher in HR/9+, OAV+, WHIP+ and (especially) ERC+, then you've got a pitcher you can plug into your rotation and don't have to worry about. (For relief pitchers, you'd like a lot more than 100+ in each of those stats.)
Of course you have to make adjustments based on various factors. If you're playing in a 24-team prog when there were 26 real life teams, a 200 inning pitcher with an ERC+ of 105 is going to be quite valuable; if you're playing in a 12-team prog when there were 16 real life teams, an ERC+ of 105 won't be so hot. Likewise you have to consider your home park, and whether you're playing in the dead-ball or live-ball era. For example, in a single-season prog 1914 Dutch Leonard's HR/9+ of 98 isn't going to hurt you much, considering no batter hit as many as 20 HR in 1914, and only a handful of batters reached double digits in HR; whereas with a pitcher like 1998 David Wells, with his ERC+ of 162 but a HR/9+ of 91, you might be a bit wary, especially if you play in park that favors HR hitters.
Granted the plus stats don't give you the whole story, but in a progressive they give you a decent quick and dirty evaluation of a pitcher's worth.