However, if you have long running requests because they make intensive use of server resources (CPU, RAM) then concurrent servers buys you very little. In that case, sending a new request to a server that is chugging along on a difficult problem is significantly different than sending it to one that isn't. That's where knowing the resource state of each server, and routing accordingly is of huge benefit.
While load balancing is a very difficult problem, with some counterintuitive aspects, it is an area of active research, and there are some very clever algorithms out there.
For example, this article (http://research.microsoft.com/pubs/153348/idleq.pdf) from UIUC and Microsoft introduces the Join-Idle-Queue, which is suitable for distributed load balancers, has considerably lower communication overhead than Shortest Queue (AFAICT the original 'intelligent' routing algorithm), and compares its charateristics to both SQ and random routing.