What are the pros and cons of the permanently-caged designs like Rapidex, and the open-style reels with removable line guards?

I have a Rapidex that I use for some close-range still-water work and for trotting small rivers.  

Neil,

My preference is for open reels and mostly use a Trudex with the line guard removed.With care i rarely have any problems with the line misbehaving and if it does,it's easier to remedy quickly with an open design.I do feel that they are better when casting directly off the reel as in the 'Wallis' style.

However i think it's a case of getting used to what you've got, and many anglers prefer the caged designs such as your Rapidex .I suspect that they may be less prone to line lifting off the spool and tangling in windy conditions.