If I were the healer, unless the undergeared tank is from my FC plus the person is guaranteed to stay in the FC in the future, there is no other circumstance I will take an undergeared tank. Why would I want to cast heal 3 times everytime the tank is hit, when I can heal only 2 times with an averagely geared tank? Whose gonna give me back the extra mana I spent?

It's reality. No one wants to do extra work.

Yes you can beat HM Titan, but sorry they used to beat it even faster and easier. It was you who put extra pressure on the remaining 7 people.

If you are so sure that you, the undergeared one, is actually carrying them back, you can advertise youself saying, "If I don't top the (insert parser name) dps meter, I pay everyone 10k GIL". I'm pretty sure you will get a group very soon. If you are not THAT confident, go gear up and play normally like everyone does.