I rolled over 186 nights last year, having received credit for 261 nights in 2015. This year, I'm only going to be in hotels about 65 nights. If I had stayed at Marriott, I would have nothing rollover to 2017. It would look something like this:
Nights Stayed: 65
Rollover Nights: 186
2016 Total Nights: 251
Expired Rollover Nights: -186
Required for Platinum: -75
Total Rollover for 2017: 0 (Since I can't go negative)
As a result, I'm staying at Hilton this year. If there were no rollover expiration, or if I could at least roll over all 65 nights actually earned in 2016, then it would make sense to stay at Marriott this year. However, since I'm losing 100% of my 2016 nights (assuming they're less than 75 in total), there's no incentive to stay at Marriott this year.
If you insist of having roll over nights expire, a better way to do it would be to let the rollovers count towards current year status first. The amount that expire should be the amount carried over less the rollover nights used to qualify for status that year. Under that math, I could still roll over the 65 nights earned in 2016 to 2017 and wouldn't be a Hilton customer this year!