I can understand why "enhanced excalibur" puts ultimate healing abilities on a 5 minute cooldown because it is a healing ability in itself.
But with "regular" excalibur, it just gives you a +8 buff to your defence stat.
It seems unreasonable for a "potion-like-buff" to put a healing ability on a 5 minute cool-down... especially when you can use an actual potion that buffs defence more and doesn't put any abilities on cooldown.
The New Wild (instanced)
<--------->
Enhancing Combat and PVP
Probably an oversight, or they didn't feel like it was going to be used by enough people to warrant changing it. Though, I agree, it definitely shouldn't be that way.