Not sure there was a previous limit on saved PMs, but I can understand the need for one. As the forum membership continues to increase, so does the data storage area.
We have a Unix server in house that is used by co-workers and at no cost. That changed about 16 months ago simply becasue we need to keep adding drive storage for user files. When there is no limits set and users are not paying a fee, they tend to not think about cleaning house on a regular basis.
This is just MY OPINION and something I'm tossing out for consideration.