Sure, MNK can tank, but it's by far not the best class for tanking.
Like Zyph said, not enough evasion, not enough HP/def.
For fun runs with friends it's certainly possible, but especially with pick up groups this won't happen.

That being said, in low level exp parties PUG is often the tank if no GLA or MRD joined.
And yesterday at Ifrit our tank dc'd so the MNK took his spot in the middle of the fight and tanked Ifrit..it was awesome
Granted, he lost hate at one point because the healers just had to heal too much with MNK just not being cut out to be a tank, but still...if our real tank made it back faster..