Time in the pits is added to their time, so a long pitstop can lose you a race and vice versa.
There is no compulsion to have a pitstop, but tank size regulations mean that most riders need at least one stop for fuel in most races. Shortened races may mean no stops, but that depends when the decision is taken..ie.. you may already have had your pitstop, and therefore you lose out compared to someone who scheduled a later stop.
Althought fuel stops may not be needed in some races, there is still a pitcrew, and it is OK to pit for a machine check etc.
Hope all that is clear...and that its accurate...I'm not an expert! Only pitted once, at the Manx, and nearly died of nerves!!