Microsoft Silverlight currently isn't used by many sites. And Microsoft has changed from trying to compete with Flash Player to now trying to compete with Adobe AIR. With Microsoft confused not to many sites use Silverlight and aren't even sure what Microsoft truly has planned for Silverlight in the end.

If like to install all plugins so you don't miss out then by all means install Microsoft Silverlight. But chances are you won't run into many sites that require it ... at least anytime soon. Microsoft needs to know what the final purpose for Silverlight will be before web developers know.

All in all to me it's better safe than sorry. I install all plugins so I don't miss out on anything. However if your one who doesn't like to install a bunch of stuff, go ahead and avoid installing Silverlight until you come across a site or product that requires it.