Suppose the user unleashes a virus that compromises company data. What does "held responsible" really mean? [Fix it themselves? Demotion? Termination?] How does this relieve the burden for the IT administrator when something goes wrong and the user cannot fix it? Most users are not as technically adept as they think they are and cannot see the "big picture" of a total computer and network environment, as well as the administrator can. I say drive the car but leave the mechanics to us!
The potential is there, but in reality it just doesn't happen very often. With mandatory real-time virus scanning and anti-spyware, independent of local administrator status, that isn't very likely.
In practice, users breaking things outside of their own machine is very rare under this arrangement--remember, they're only administrators on their own machines--and doesn't justify the company-wide slowdown of IT-only administration for all Windows machines.
This frees the admins from having to do everything so they can focus on stuff that really need their attention. Users who aren't capable/comfortable installing their own mouse can always call an admin to do it, just like always. They just aren't forced to call him when they are able to handle it.