It would be awfully hard to not pit once in a 25% race, and be competitive enough IMO. Using Hards on most tracks sounds somewhat okay, but you can't push hard at all. Like Stuart said, circuit wear and a heave fuel load would probably be a burden for an entire 25% race distance.
In the league I'm in (Currently named The Jenkins Cup), we have a rule where you have to pit once, making the races more exciting.