This is useful on both attack and defense, if you need to assault the enemy it's realistically not going to kill that much (always a lucky 6 on the fire power though) but the pin it provides is priceless, whether to silence artillery or pin infantry before an assault it has a use. The main issue is the relatively short range, often needing to advance into range to be effective. On defense it can break or pin enemy infantry moving up towards you, pin support units like artillery, or even force a bail on TA0 units like half tracks.
Why do you need to have a good skill rating or high firepower when you can fire smoke? This is often my most common use of mortars, and it's so good! With four attempts to range in, add in a little recon lifting GtG you have a really good chance of laying smoke to cover your advance! Conversely if there are some long range enemy units shooting at you - take them out of the equation - I also like dropping this on spotters, especially as many players keep them on towers quite far back, but stopping a repeat bombardment sometimes is all you need to force the enemy to fail to range in next time.
I will go out on a limb and say this is the trickiest way to use a mortar well. I tend to only use this if I'm on the attack, this brings about the mortars biggest weakness, 5+ save and no gun shield, now you can move quite quickly as its a man packed weapon, but you are vulnerable! Once in position though it's very useful, got some HMG's to remove - sure go ahead, a 3+ firepower lets you pick your target and hopefully remove them - they're also not bad against bunkers and nests either.
Ah, probably the most finesse use of mortars, it works on attack well - but defence is my favourite! You pair these beauties with some AT guns, open fire on your opponents tank and smoke the sources of return fire (unlike smoke bombardments these do not have to happen at the start of the shooting step!) now this is not completely effective at stopping return fire, but it will mean the opponent is either stationary and you count as concealed and GtG, or they move and drop their RoF. Not a shabby use of them.
Well as always thank you for reading, and until next time.