I don't think that spark plugs in particular will be a problem on his diesel...
i dont know if this helps but i have heard afew complaints regarding water leaking through from the windscreen jet washers and dripping onto the engine and causing problems eg. water in spark plug sockets. the engine cover would eliminate any risk of that happening.
could save you alot of hassle in the long run. but im not 100% sure so please correct me if im chating complete nonsense lolol
However, there may be some small truth in what you say