Regarding your second edit: If anyone actually tried to run Apache/mpm_prefork with "MaxClients 150" on an average Linode or cheap dedicated server, they'd OOM

Regarding your second edit: If anyone actually tried to run Apache/mpm_prefork with "MaxClients 150" on an average Linode or cheap dedicated server, they'd OOM and start thrashing as soon as someone started requesting a couple dozen PHP pages -- even if the rest of the requests were to static resources. That's just another way to get yourself DoS'd. Received wisdom on the Linode forums is that you should handle no more than 10-15 simultaneous connections if you're stuck with Apache/mpm_prefork and PHP. So your original point stands.