It's a d20 system. So, 60 is a magic number (miss 95% in GH), so -20 means anything over 40 AC counts. An AC of 30, unfortunately, is not much better than an AC of 10.
For the most part, if armor won't let you hit >50, robes are good for the extra effects you can get on them, and you can switch them out pretty fast (for multiple useable effects). If you have the Reaver's Refuge pack, this is a good time to look into getting some dragontouched robes/outfit. If not, here are some things that might help on some robes:
deathblock
fortification (if not on an item)
greater fire/ice/acid/electricity resist
haste/rage/fog/... clickies
hammer/spear/axeblock
"shadow" (for stealth builds)
command
....
The list goes on. Basically, robes can fill what you don't have in your equipment. Remember to throw that armor back on when/if you're doing favor runs tho
.