Some moderators will specify that eliminating is compulsive, thus they cannot No Eliminate. Frequently this is for balance reasons, or to break deadlocks (e.g. Happily Ever After situations) and thus force the game to have a definite ending.
Whether it is appropriate or advantageous to No Eliminate is situational. Players may No Eliminate in order to maintain an odd number of living players, which is mathematically advantageous. Alternatively, they may vote No Elimination in hopes that an investigative role will get the information needed to eliminate more accurately or efficiently.
In some games outside mafiascum.net, the density of power roles (especially investigative roles) is high enough that players will simply refuse to eliminate until a Cop has enough information to step forward and lead the game. On mafiascum.net, it is typically the opposite - in particular, there is no expectation that there is a single power role strong enough to give the Town a major advantage if used for a few Nights. Besides, that kind of strategy only works if that power role avoids getting killed.
Note that when No Eliminating, the players who die overNight are chosen by scum, so the benefit that Town gets from losing players at a slower rate is not as good as one may think.