You might have heard about Paul Barnett's choice to discourage his designers from playing WoW. Not everyone likes the idea.
As I said at Ryan's, odds are that most of Barnett's team played WoW before, so it's not turning a blind eye to ask them to not play WoW now.
It's a trade-off. Fresh isn't necessarily better, but avoiding too much exposure to other games will help the designers to approach things from the fundamental goals and pressures, rather than starting with models. As Paul pointed out, the fact that something was extremely successful for someone else doesn't mean it will be successful for you. In any work comprised of many elements, it's how things fit together to create a whole that matters most. And in any art, masterworks reflect the individual designer as much as shared wisdom.
Beethoven wrote his best work when he was deaf. He had been influenced and trained by many other composers beforehand, but the silence allowed him to explore in a way he couldn't have done otherwise.
As with most things, balance is good. Designers should alternate between periods of studying others and periods of quiet seclusion (from other games). One truth every world religion pinpointed thousands of years ago is that periodic silence leads to clarity. It's harmful to cut out inspirational influences entirely, but it's good to moderate them with peaceful reflection and lone exploration.