There are a few BIG reasons I can think of why bst almost always dual wield:
1. There a no good pet buffs on shields.
It would perhaps take 3x shields to come close to what we can do with dual wield. a 10% pet -pdt shield (or -damage taken), a Pet Magic Attack bonus +30 shield, and a pet Attack + 30 shield. even this wouldn't compete w/ the tp bonus 200 and pet mab +25 kumbhakarna. but it would be close. if there were 1 shield that might make it worthwhile, it would be ready recast -5 seconds to replace Charmer's Merlin.
2. Master DPS. Dual wield is 100% necessary for faster tp build and more frequent skillchains. If I could self-skillchain 1 handed, I probably would.
3. Charmer's Merlin - the 5s ready recast is invaluable for bst dps. It means that we either swap weapons for pet ready moves and don't engage on master at all, or we keep it equiped for off-hand.
The only situation where a bst shouldn't do this is if using a pet that is all about utility instead of dps, and ready move frequency is less important.