i too will go against many on this thread and state that you really do want to keep one in place.
I do agree that its not really for sump protection if you are going to smack it on something, but it will provide protection from stones/ debris and water!
The cambelt for example is quite open to the elements from under the car without one. Some pipes, electrics etc... and add to that there will be lots of water and salt for the next 4 months.
They are not cheap though. (about £100) and on balance as seen on this thread alot of people dont have one fitted.