Well obviously it needs a little thought. It should have a limit, a maximum of APs converted to defensive bonuses, but I don't believe it should be combined with the ability to save APs. It should be one or the other. Getting stuck in a cycle of not being able to use high AP skills can be avoided by not fortifying. It's a strategic choice, fortify and improve survivability, or stay vulnerable as you prepare to use a powerful ability.
I don't think having both defensive stance AND AP conservation would make the ability balanced. If able to defend AND still save APs, then just throw away the option, and have "every AP saved improves defense until used" a regular feature. Not convinced it would add value to the combat system. Strategic depth is about choices, in my opinion.
