I think Neil brings an important fact to light regarding mortar attacks - or more specifically, opening a combat encounter with a mortar attack. The falling rounds give little or no warning, giving them the advantage of ambush - ie. taking the targets by surprise. I'd also have to agree with Neil's suggestion that falling mortar rounds in the midst of an already-hot combat kind of loses some of its impact since the targets are already taking some sort of evasive action if possible.
I think this probably should translate into an increased chance to pin, suppress, or kill with the opening salvo (which might only be one round). Going beyond mortars here, I think the ambush concept of applying bonuses to initial attack rolls for any kind of fire ought to apply - and this thought is probably not a stranger to most rules systems.
Another point Neil mentioned is that for game purposes one side shouldn't be allowed to simply pound the other with mortar rounds right after another - ie. the targets should get a chance to act. I agree with this certainly. In an I-Go-You-Go game this can be done by simply requiring a check for suppressed figures to take some action (maybe at some reduced mobility). I kind of like how Arty Conliffe's Crossfire handles one-sided pounding - if the attacker can't achieve significant results (ie. suppression or kills), then the targets get a chance to do something besides take a shellacking.
Finally, I think skirmish games do benefit from interrupting actions - things that come under such names as opportunity fire or overwatch - and I believe mortars ought to get those kind of options. I think this mode of fire typically applies only to direct fire, but I can really see it working for a local mortar team, self-spotting or with a spotter.