"Naoki Yoshida: Again, from a technical standpoint, it’s not impossible to do. But that said, looking at the jobs that are available throughout the final fantasy series, it would feel awkward say if a Black Mage jumped into an instanced dungeon with full plate armor. I think we want to maintain the image that we’ve depicted through these different jobs that are available in the series."
It continues though.
"That said, Final Fantasy XIV has been around for 10+ years at this point now and we understand that glamour and having different gearsets in terms of appearance, cosmetic elements, is a big part of players’ play style, and we totally understand that. So, there have been, in passing, conversations that would be brought up amongst even the team mates saying, “Should we consider opening it up?” “When should we do that?” Of course it’s still not anything that’s set in stone, but we do want to look to having more freedom for our players for sure."
This is taken from a Q&A from the Media Tour, found on Gamerescape
Final Fantasy XIV: Dawntrail Q&A with Naoki Yoshida