Negative Affects:
-You get a loss.
-You lose training minutes.
-You lose game shape minutes.
-You lose experience minutes.
-Your fans probably won't be happy, which may lower future attendance and merchandise sales.
-Your opponents will probably be pissed at you for screwing up their training system and game shape minutes.
-Smart managers will not accept or send scrimmage requests to you, because they're afraid you will forfeit.
Positive Affects:
-Unknown.
So, why forfeit games? You have 10 players, which is plenty to play 2 games per week (I'm assuming if you forfeit games, you dont really care about scrimmages). You're in D4, which is a good time for training players--especially if you're in last place and have your sights set on the #1 draft pick instead of avoiding relegation.
If you accidentally forfeited games, try setting a "default lineup"--this will make it so if you forget to set a lineup, your default lineup will play (assuming your default lineup has at least 3 players).