Most discos will run fine without it, they have a very good cooling system. Many people keep it in the back for the exceptionaly hot day or for towing. You can refit, the only benifit of not having it is a very slight increase in power and it stops mud being thrown everywhere when off roading!
It's worth carrying it with you when possible in case you need to do heavy towing or do get an overheating problem or go offroading on Salisbury plain.
Took mine off a few months ago seems fine, the only time it got hot was when i was greenlaning in low range so it was revving quite high just stopped for a few minutes then put it back in normal range and was fine.