Both NiCads and NiMHs give practically the same voltage through the whole discharge process, alkalines show a steadily dropping voltage. Now, while it's nice to have good, bright light through the whole charge, you can't see if the rechargeables are full or if they're five minutes from empty by turning on the light briefly. With alkalines you notice a clear reduction in light intensity when they're close to empty. Also, NiCads have a noticeable self-discharge (NiMHs less so), while the self-discharge in alkalines is very, very low. Self-discharge isn't much of an issue for cells which are regularly drained and recharged, but it's very much an issue for cells which are just kept for backup.
I don't know about GUE, but those are the reasons that I prefer alkalines in my backup light. I can't charge those cells without opening the light and taking them out, and that's just not happening before every diving day.