the best answer i've seen is that USF mortar was not needed but it is a nice addition for diversity. of course current OPness has to be dealt with.
i also think budwise has a really good point in saying why he thinks it should belong in a tier: RM > gren or volk.
but i think that will be too late for a mortar. i want to revive the idea of USF tiers divided into two sub tiers where vehicles would be locked behind another paywall. in my head both lieutenant and captain would be 30 fuel each (obviously BAR and zook should be looked at), and will come with mortar and 50cal / at gun and packhowie while m20 and m15aa / stuart would behind 100mp + 20/30fu paywall.
Dunno, but what is needed right now is a corrective patch for the mortar. USF mortar version that was in the balance mod wasn't game breaking at all.