Steve of course the glide is OK on a portable, surely you can be the judge of that when using it?!!
It will theoretically lower the CFMs of the system a bit because it restricts the flow, but when the wand is on the carpet it isn't pulling that much through anyway compared to open-flow. Crucially it will not affect the lift. Your main benefit is the reduced fatigue, but on some carpets it will help draw a bit more water away compared to no glide. Surely you have noticed that it makes your wand technique much smoother?
I'm not convinced a glide leaves all carpets drier, some maybe, some definitely not any drier but certainly no worse than not having a glide. Expensive but worth it just to lose the back ache.