It's been discussed on AH.com already. Other jobs doesn't get the option to lower DPS to boost defense, outside of defender for WAR and /WAR people, which BLU can do too.
You sound like every BLU cocoon full time and lose every parse because of it, but the fact is they have the choice to not use cocoon and maintain top end DPS, and choose to use cocoon when the situations call for it.
You can't give out MG full time if the fight last too long or repeat pop, like you previously mentioned....longer fights. Unless you invite 2 BLUs or something, again, like AH discussion,
this creates the scenario of MG mechanics making 2x BLU setup a lot more appealing thus filter out other melee DDs. the bold part is pretty much the main reason why BLU debate is being brought up on AH over and over.
There no real downside of using BLU, and their DPS is definitely not as bad as all the BLU main make it sound like unless I do something seriously wrong with WAR spreadsheet, or unless BLU sheet has incorrect ws forumla. If the situation calls for more defense, BLU can do it. If the situation calls for all out offense, BLU still keep up with 2h DDs. That's what people meant by "BLU has no downside", they don't necessary mean BLU can set every offense/defense job trait at once and cocoon full time or something. They simply mean in situations that need a melee DPS, using a BLU over other jobs like WAR MNK DRK SAM DRG has no penalty, while in situations that needs more defensive DD, BLU did way better than almost every other melee job except tanks.