They shouldn't implement the MX style. MDI does not cope at all, or copes very poorly, with multihead setups. In general, applications should not be handling window management. That is a task for ... window managers!

People complaining about gimp's multi window interface usually lack a decent window manager. More often than not, they live in MS windows, using prehistoric window managers.

If you use devilspie or KDE, I'll be happy to send behaviour scripts that produce an MX-like interface in multi-window apps like gimp.