depends how you see it, you've come a long way to just give up 25k points. i would book at a low price hotel and jst have a mini vacation. 25k could be equivalent to spending $2,500 at marriotts (not including bonuses) or around $300 if you buy them from marriott. But you got till next month to meet the required nights
So you're 2 nights short of getting the final 15,000 points on your megabonus and are asking if booking a 2-night cheapo weekend would be worth doing to gain the points.
15,000 points will, when redeemed get you 2 nights in a Cat1 hotel. Hence if you book 2 very cheap nights (at say $100 for the stay - which will also garner 1,000 points for the stay itself) anywhere on the Marriott system you should get back more value than the $100 paid out, namely 15,000 points redeemed at the right Cat1, on the right dates could easily be worth double what you paid out. But, do you REALLY want to spend all that time at a Cat1? These aren't Marriotts luxury hotels, they're in Cat1 for a reason, they're the most rarely redeemed destinations, that says a lot!
I'm afraid after that it becomes more difficult to see the value of your proposed "mattress run". 15,000 points will get you one-and-a-half nights at a cat2. Slightly better locations perhaps, but by no means leisure places. The maths here is likely to be very marginal. 15,000 points will also get you 1 night at a cat3, all the same your 2 cheap nights will have to be really cheap to make 1 night at a cat3 stack up.
Generally speaking I value MR points at 1 Eurocent each, so 15,000 MR points are worth about $200. Generally in the US you'll find this value easily obtainable on weekday Cat 1/2/3 redemptions, tougher at 4,5,6 and 7 (the category rises are not matched by the room rate differentials) but at Cats 8&9 that Eurocent should be easier to obtain on weekday redemptions. In Europe Cats 1&2 are rarely found, but redeeming at Cat1-5 for weekday stays generally sees that 1 Eurocent. Cats 6 & 7 remain tough, Cats 8 & 9 are generally good value, for instance I recently redeemed 2 nights at the Cat 9 Paris Champs Élysées, €600/night, a value in excess of 1.5 UScents per point, if I'd redeemed the following night it would have been €800!
Youll find you could redeem those 15,000 points for $250 (with difficulty in the US), or as little as $50 (stay at a Cat 6 - 30,000 points - on a $100 weekend night!). I certainly would'nt do the mattress run for 15,000 points, I couldn't buy the nights cheaply enough here in the UK, but, if you can get the nights cheaply and have reason to use an expensive Cat1/2/3 hotel, you could make it pay...