'Mechs like the Crusader, Catapult, Trebuchet, and Whitworth aren't entirely "long ranged support", they're quite capable of functioning as brawlers once the LRM ammo is gone. As dual-purpose designs, they don't need to carry quite as much LRM ammo, and may actually benefit from running out before they start to lose too much armor. A standard Archer, on the other hand, doesn't carry enough secondary firepower to be an effective line 'Mech (a 40T Whitworth packs 50% more short-range firepower than a 70T Archer), although it's weight makes it viable to some degree as a melee unit.
As said, in a campaign situation, you may want to carry more ammo than normally needed for one battle. That, of course, is assuming that the first battle leaves you with enough armor to use that extra ammo in the second fight before you explode. Having twice as much as you need is probably overkill, unless you're expecting a series of easy fights where you don't take much return fire, but need to take out a lot of targets. In most cases where you hold the field at the end, you're only going to hold it long enough for your tech teams to salvage as much as possible, or until some other unit arrives to relieve yours, then you head back to base for reloads and re-armoring. You want a couple of rounds left in case something does show up to contest the field, but usually the enemy has already spent all or most its available forces in the area, and is retreating back to its own base for repairs and reloads.
Then you have the occasional battle where you've got to finish off some target in hard cover at bad odds for your ammo-dependent weapons, from safely outside of their gun range. Basically, if you can sit there and plink away, there's nothing they can do to save themselves without coming out of cover, so they're just hoping you run out of ammo and have to close, where it will be a more even match. In such cases, there's never "too much" ammo.
Also consider the RFL-2c model (NOT a "IIC" Clan design), which mounts dual AC/10s and respectable armor, but no secondary weapons. It WILL run out of ammo, but there's simply nothing that could have been sacrificed to add another ton without seriously compromising the design in some way. I want the amount that it has, only because no more will fit.