Zerker and medic obviously are going to want to use off-perk weapons all the time, but there are other situations where it's useful.
M79 with combat shotgun and AA-12 is my favorite support loadout, the only way I'd take a hunting shotgun instead is if I were going for the melee/HS instant kill on scrakes, and I find that technique too finicky for regular use in full HoE games.
Commando with SCAR and M32 is great in small HoE games, you can easily solo fleshpounds if you catch them farther away and although commando is known as the anti-trash perk he really has no way of breaking out quickly like a support player unless he brings a grenade launcher.