The LB penalty for doubling up on the same jobs is far more than marginal, considering even one double job in Byakko Ex means you won't have tank LB3 for the second tiger (and people don't want to do the mechanic properly so, there's that). Tank LB2 isn't enough even when the tiger is at 50%. In Savage weeklies, people don't allow doubling up on DPS roles to avoid competition for loot--e.g., they say BRD or MCH, instead of allowing both, because they have to compete for gear. PF is made with the intention that party leaders can set whatever preferences they want for their party. You can't punish a person for saying "I don't want to double up on jobs, because we will get LBs faster" or "I don't want to allow multiple casters because they will all be fighting over caster pieces if they drop, and we're in a lockout right now". If you don't like the rules set by the PF leaders, make your own with no such rules.