Ice is a great type on gym offence, tearing apart dragons (most are doubly weak), Togekiss, Drifblim and both Exeggutors. Looks like many legendaries (especially in Gen V) are weak to Ice.
Of course, Mamoswine is strictly superior to all other Ice attackers in both DPS and TDO, but there are specific matchups where it is not the best Ice attacker. Could we identify them? The obvious one is the Exeggutors in gyms, as Mamoswine is weak to their Grass moves.
I have maxed a 96% Mamoswine, taken another four to level 30 and am seeking another 96+% one to max out, but if there are enough relevant matchups, I may consider maxing a Glaceon as well (I have multiple 98% Eevees) or adding Ice Beam to my Focus Blast Mewtwo (but 100 RC is a lot). Unfortunately, I have only one Weavile, with dark moves and IVs not worth maxing out.