I have owned hot tubs for several years now, and sure usage goes up and down. I bought mine to be used mostly in the winter time. I live in New England and hotubing in the winter is great and it get's used a few times a week. And friends love comming over and usign at well. The key was having it close to the house. No one want's to treak across the frozen tundra to get to the tub.
During the summer usage drops off, as we prefer to cool off in the pool, but it still gets used now and then. Espically after some heavy yard work and the muscles are aching.
I see people neglet and not use thier hot tub because it gets too $$$$ to use (Ie they bought a cheap spa with poor insulation and crappy therapy and don't keep the water clean).
I think you need to ask yourself why you are buying the tub? you mention you have an IG pool.
IF you are buying a hot tub as an acessory to your pool, then sure, I could see it not being used that much, (although jumping from the hot water to the cool pool water is inmvigorating).
A hot tub is very different that a pool so if you think that since you already have an IG pool. you wont use the otehr, I disagree. The pool is larger and can be used to float in, exercise, cool off and many folks can use it. In NEw england, the pool season is only a few months long. Hot tubs are mostly all year.