It depends on what you are trying to be flexible for. If you want IDF and special munitions then only LRMs will suffice. If you want Flak the HAG covers that better. It should be noted that HAGs have less working against them then do LRMs which can be diminished by AMS and reactive armor. While there is armor that will half gauss damage it is pretty bad and are less likely to run against it. The HAG 40 is the best of the bunch, but HAG 20s on the right chassis aren't okay, but nothing to write home about.
Doesn't LRM have flak ammo? LB and conventional autocannons also offer anti-air capability.
I might feel better about them if I played them more but I'm mixed. They are a LOT of weight and space, tied to a tiny ammo bin. 16 tons of gun, and 3 rounds per ton of ammo. I'm going to VERY averse to firing at anything beyond 16 hexes, and not wild about taking shots past 9, unless I am packing 3-4 tons of ammo. That's 20 tons and 14 crits.
Figuring on cluster rolls of 7's you will average 24 points of damage, grouped 5,5,5,5,4. When I'm probably shooting at an assault mech, that doesn't give me warm fuzzy feelings.
On the other hand, pair with an ER PPC for some long range fencing, then I'll get interested.
I've used them seldom, so take this with a grain of salt, but baring luck they've never impressed me. They peel armour. They don't dig big holes or generate enough hits to truly critseek.
The best use I've seen is on the Thunder Stallion; as the second weapon fired, after the LB20X.
I think Greatclub is right. They don't generate enough location rolls to kill by TAC/floating crits/head shots. SB Gauss is going to average 9 locations per hit.
But like I said above, when used as part of the hole-puncher/hole-finder combo they have potential.
I'm more interested in the HAG 30. The 20 will give you 4 location rolls in close, 3 at mid range, but only... Cluster roll of 7, -2 for HAG at long range is a 5, for a 20 point attack is 12, which is the same what it would be for medium range. Huh. The 20 is looking a bit more appealing.