I would encourage you to develop more pitchers for the following reasons:
1) It gives other players on the team more touches and keeps them interested in the game.
2) As you pointed out you have lost your 3rd starter for the summer...what if you lose #1, #2 or both?
3) You may have a diamond in the rough who just needs to be developed through teaching and experience.
4) If you play the same teams often it gives them a different look and disrupts them from guaging your regular starters.
5) You have more pitchers!
Pitching development may bring some short term pain but may be worth it in the long run. My youngest son's team won a championship because his coach developed pitchers...it hurt them early in the season but when they played 6 games on a weekend for the championship. The team we played ran out of pitchers and that was the difference.
As for your dilemma...well, I'd still keep an eye on pitch count. If your strongest pitcher is throwing minimal pitches/inning then you may want to take him to 3 or 4 innings. If he throws 35 pitches/inning for 2 innings then limit him to 2 innings. If your up by a lot of runs then see if you can squeeze some extra innings out of the weakest pitcher. If there is no tomorrow then you just have to go with your best.