[quote name='CaseyRyback']Unless you are using sanitizer sticks there is no reason to drain and refill every month. You aren't backwashing enough, balancing your water properly, or there is something

ed up.
As far as the chemicals putting strain on the filter, as long as they are broadcast in the water it shouldn't cause any problems. The harshest chemical is going to be the chlorine and it burns off in the sunlight (as does the cya).[/QUOTE]
Sure there is a reason, it's because I chose to do so to save on chemicals for both expense and exposure.
Most manufacturers recommend that the hot tub be drained and cleaned every 3 to 6 months anyways.
I chose to do it every month since it doesn't cost that much for the water and my tub is very efficient so it's not a lot to heat it back up.
This also keeps me from having to balance the water which is also easier on the equipment as well as the people who use it.
If the hot tub is used very seldom then sure you can balance the chemicals at low cost and little maintenance.
Mine, however is used almost daily (5+ days a week) as I'm on my feet most of the day for work.
Oh, just FYI you don't backwash a hot tub - (that would be a pool) and I would hope they have a cover so to help keep the heat in the hot tub.
Again, just my $.02 = OP should maybe do some reading/research and decide what's best.