User blog comment:FairyRaven/Fairy Tail Chapter 365: Fairy in the Jail!, Review and Commentary/@comment-7418318-20131228133628/@comment-7418318-20131228141047

Actually it makes a lot of sense to have her stripped.

For starters there's the psychological level. No matter how thin, clothing will still provide you with a certain feeling of safety and most people are embarrassed and humiliated by being naked in front of strangers. Both would undermine their resistance to questioning.

While Erza seems to be pretty much immune to the latter, her only feeling save in armor (while much less pronounced than before the Tower of Heaven, it's still present, or she wouldn't be wearing her armor quite so often) would make the her more susceptible to the former.