Ignore PFs like those; they usually turn out to be traps anyways. People that exclude jobs for this type of content are generally the kind of people you would rather not party with anyways. Think of it as you dodged a bullet with that group. Neither of the current Ex trials have a DPS check so tight that everyone needs to be in the 95th percentile in terms of damage. Unless your entire group has terrible DPS, taking a MCH isn't going to cause the party to instantly fail mechanics, or reach enrage.

I was in a Lakshmi farm earlier this morning with a MCH that was consistently second on the DPS list, after a NIN. They were parsing around 3.2k DPS consistently, and that wasn't even with 50% Balance uptime. I know they could stand to be higher (we all know MCH needs love; I'm not saying that they don't need to be looked at by SE and rebalanced), but the player makes the job. SAM are supposed to be the strongest out of all DPS jobs, but I've been in groups where there have been SAMs consistently at the bottom. The job one plays means nothing if people don't know how to play it properly.