If you have the money to invest in high availability options, you should. Clustered servers, mirrored instances, etc. are all good options for mission critical databases. And, as Grant said, it's more of a headache to have those databases sorted out into different instances or different servers. On the other hand, if you can afford to have multiple servers to spread out the load, it allows for a certain degree of CYA because losing one server means you won't lose all of your mission critical databases at the same time.

But it sounds like you don't have the money or the employee bandwidth to handle something like that. So what you need to do is group them together for easier maintenance and make sure you have a kick-butt disaster recovery plan to go with it.