FWIW, Charlie Papazian (in response to a letter to Zymurgy I think) said he doesn't get why you would do it, would not do it himself, and doesn't recommend doing it.
Main reason I have heard is to reduce carmelization, especially in lighter beers. I really don't get how would you get better hop utilization. If you boil hops in water, you get way less utilization. I brew all grain, so am I getting worse utilization? I don't think so. It seems you would get WORSE utilization with a lower concentration of wort. Just my opinion, I'd love for someone to chime in with a more scientific argument rather than just opinion and hearsay...