The only downside to offering a start to an ineligible player is the very slight chance that he passes his SAT's and becomes eligible before recruiting ends. If that happens, he'll obviously be expecting to start. If he stays ineligible, then you're off the hook because promises don't carry over to the following season, but you still get the credit for the promise during recruiting.
One other thing to remember is that there are no eligibles in D3. They can play immediately. The only way a player can become ineligible in D3 is due to his grades during the season. Short version, if you recruit (dropdown or pulldown) an ineligible player from D2 to D3, he'll play immediately and any promises made during recruiting will be expected to be honored.
I hope that wasn't too confusing and helped a bit.
4/21/2012 2:32 PM (edited)