having two great drivers is fine if they get along. But with Alonso threatening to shop the team in and the pair fighting at a detriment to both WDC & WCC it's not surprising one went. They do want two great drivers and I'm sure Hamilton doesn't mind a challenge.
In 1989, Ron Denis was able to keep both Prost and Senna, despite them not getting along.
Management of drivers is an important skill and in the late 80s and early 90s, Denis was the best at this. (Frank Williams was notoriously bad at losing his World Champions and being unable to re-negotiate contracts).
Even when one of his drivers publicly announced that he wanted out of McLaren asap and wanted to drive for Williams, for free, Ron Dennis persuaded him to stay in 1993. And even when Senna moved in 1994, Denis stated that he will do everything he could to get Senna back, by improving the car. Now that is what I call a manager/boss who is prepared to do everything to win, ie. employ the fastest driver, no matter what.
The Ron Denis who got rid of arguably the best driver in F1 in 2007 is the not the same guy who did all he could to keep hold of his drivers in the late 80s and early 90s.
With regards to Hamilton liking the challenge: I believe that he would much rather have a slow driver like Heikki, than somene who is capable of beating him and making him look ordinary. Most top drivers are like this (MS, Alonso, Prost, etc). The only (top) drivers who come to mind, who were prepared to go up against the best were Senna and Mansell.