You can use exit interface in your static route command, when you have point to point links. You can not use it in multi access interface (like ethernet, where there are lot of machine connected to the same segment). You should also NOT use exit interface in the static route, when you have configured subinterfaces in a physical interface. This is because in these scenarios, you can not uniquely indentify the destination by specifying exit interface (sicne there are more than one devices reachable through that interface).
There is one interesting observation about use of exit interface in static route. When you use exit interface in static route, your router considers that network as directly connected network (cost to reach that network is taken as 0). So a static route defined using exit interface is always preferred to a static route with desitination IP Address.
Hope this helps.....
