The time limit is to prevent site systems from being abused - for instance, may websites are vulnerable to Denial of Service (DoS) attacks, where a web robot can be used to make hundreds of requests per second on a given function (such as PM or search) thus causing servers to become overloaded and the website to become unavailable for everyone else. Time-limiting certain functions like this protects against these DoS attacks. The PM time limit is also there to prevent an indivudual from registering and then using the PM system to spam other members.
RYL v3 contains many of these safeguards actually on all sorts of functions, many of which members will never discover!
