As dk240t points out, you do lose out on real benefits for the time you are PM so, in my view, it is only worth it if you know you can hit DM again very quickly.

I did this a few years back - I was just a few miles short so I rolled over almost the full 50K. I made DM again by May the following year and have been carrying that rollover, more or less, ever since then as my typical annual mileage is around the DM threshold. The rollover came in handy this year when I flew a lot of short flights at the beginning of the year and probably wouldn't have made DM otherwise.

However... I probably wouldn't do it again. That was before MQDs and that changes the calculation. MQDs don't roll over so you need to consider how quickly you can spend up to the DM level.