My first large scale web application was a webmail service built in C++ (!) where I early on decided we'd ditch nearly all freeing of memory, as it was running as a CGI, and it was much faster to just let the OS free memory on termination. The exception was for any particularly large buffers. Coupled with statically linking it, it reduced the overhead sufficiently that running it as a CGI performed well enough to save us the massive pain of guaranteeing sufficient isolation and ensuring we were free of memory leaks.

Especially in a request/reply style environment, long running application servers is largely a workaround for high startup costs, and it's only a "bad idea" in the instances where removing that high startup cost is too difficult to be practical. Overall I love avoiding long running programs.