You can...technically. What are you trying to do? There are several threads on this topic.
Pro: View activities across all your websites for your leads. Action based on interest in different product lines with their own microsites.
Con: None in most cases. Heavily loaded Mkto instances will not like more traffic, but this is getting better.
Caveats: Activities are anonymous on a given domain until a lead is known on that domain. Significant development is necessary to skip that requirement.