Not necessarily. I'm not aware of there being any benefit to having two medium spec servers versus one high spec server. I think the reason some people choose seperate servers for Apache and MySQL is either price (since you're paying a premium for a top-end machine); or because a single machine (even a high-end one) isn't powerful enough for their needs, and splitting Apache onto one machine and MySQL onto another is the logical (and easiest) way to spread the load over the two machines.