I had an experience like this too recently. I was out in San Diego about a month ago and stayed at Harrah's So Cal. I have a different host assigned to me there than my normal host who I deal with for CET and don't really know her but asked her about getting a room. I was getting offers still from playing there about a year and a half ago, but they were free rooms every night except Saturday. So I was given the usual "come see us before you check out and will review your play, always do that". I almost always play enough to warrant a free night so figured no problem. I had Friday night comped anyway, but not Saturday.

On my first night there, I had the usual host on duty come up to me when I started playing. I'm Diamond so this is typical of any CET property i go to. So I asked him about this too, thinking maybe he would be there when I check out so would be good to have talked with someone. Gave me the same story, we would be happy to review your play, please come see us.
But then I asked him if they use reward credits. He told me yes, that he has to use all of them first. I even said what if I play a lot and get like 25,000 tier points in one day, he said even then he would have to use my reward credits first. No exceptions to that. He also told me they assume when they offer things to players that they don't have much or any reward credits since most players use them all up. Then they can give "discretionary comps" out.

I'm one of those people too who like to keep a nice bank of them, I had $1900 worth before then. I keep them thinking I'll get something nice out of the catalog, or its nice to have them if I have a really bad gambling day and can't get more money, its like an emergency savings account at the casino. So I had to use $150 of them for my room, not a huge deal, but lesson learned. I could have used that as freeplay if I didn't bank them up. I know casinos would rather players not bank these since they have to consider it a liability for accounting purposes, so maybe that's the reason why?

So yes, better off just using these up.