If my defense and magic defense stats are both 255, will there be any benefit to shell and protect? I'm trying to decide if I should build armour with auto-shell and auto-protect for my soon-to-be-maxed-out characters.

There will be an effect,yes,but they aren't really neccessary if your strong,I would say protect if you are going to put one as auto,but yes,even if your stats are maxed,you will be able to see a difference in the damage you take,Protect ans Shell will still reduce damage if your stats are maxed.

I don't see it to be really necessary. With high HP and 255 defense and m.d., you won't take as much damage... However, it could be useful fighting some Monster Arena creations and/or the Dark Aeons...

And yes Protect and Shell would still work w/255 stats...

It will still work since 255 def/mdef doenst make you immume to it.

Shell/Protect DO NOT affect your stats. They affect the damage received by usually halving it :)

So they're always useful, but not always necessary...

i find shell to be pretty useless except against yunalesca and seymour .... but protect is always tha shyt.