Does creating addional app pools add much overhead to IIS?

In my SharePoint setup (2010 and IIS 7.5), I've been told it's smart to create separate app pools and then run those app pools using separate accounts.

Upon research, I've had suggestions to not bother with this because -

a.)  It reduces performance and

b.) Doesn't offer a compelling security improvement (*I* disagree)

Does creating separate app pools (I don't think I'd be creating more than 6-10 at most) and then running them as different users create much of a performance degradation (disk, memory, and/or CPU) in IIS?

Ed