The problem with this is in the PnP spell its a +4 armor class, as in you're now wearing the magical equivalent of mundane scale mail w/o any of the penalties. It is not the same as wearing +4 clothing or bracers of armor +4.
Imagine that the wizard picks up a shield that gives +4 to his AC. Its not "armor modifer." This way it wouldnt really help to cast the spell on the fighter, since his 8 AC from full plate is much better than the 4 for mage armor. Basically the spell replaces armor, but any +1 or +2 on clothing or bracers will still stack. Only good for the mage and maybe a light armor wearing companion, and not overpowering.
However, the only way to do the spell in NWN would be to give a +4 "armor modifer" effectively making the spell an overpowered magic vestment. Thats why its the way it is in NWN. Personally I like it, it starts out strong, a major help at lower levels, while at higher levels its effectiveness is minimized (likely down to only a net +1 AC) but it isnt useless. Its still +1 AC.
-TV