Would you say the same about professional athletes or chess players?
I've to admit that I don't think that I can claim to have made some significant improvements for all of humanity so far..
Every person on the planet needs to be entertained, and since there is someone out there who does make significant contributions to society that does like Starcraft enough to watch a documentary about a popular team, it must have some inherent amount of worth.
Being an entertainer isn't the same thing as curing cancer or engineering a cease-fire between warring nations, but it doesn't have to be.