Exactly this is a special case were a cluster weapon is better. That's not limited to the Mass Cannon. Motherlode and Matron are two other bosses where it makes sense to use a cluster weapon. Why? Well, the secondary explosions doesn't count towards to the damage limit you can deal before the next boss phase triggers. Actually a bug...
I do have a quite good Mass Cannon with Cannoneer synergy, enhanced magazine and fast reload. So I'm not jealous. It doesn't compete to my FRC Big Boomer / Sovereign Grenade Launcher since I'm always ahead. Mass Cannon users love to shoot one meter in front of me. The visual effect is awful, so bad that I'm changing to Mass Cannon and shooting back in their face. Then they usual quit using it :p

