I think it's a good idea to understand how to use your computer and that includes how it handles a dive which exceeds NDLs. Beginner divers can freak out a little when, due to poor dive-planning or some unexpected event underwater, their dive computers enter into deco mode. It's never a good idea to push NDLs (after all, that's not diving conservatively), but the last thing you want to do is freak out because you don't understand the display on your computer.
There are certainly things you can do to mitigate risk while putting your computer into deco. It just takes a little planning. Why not switch computers with a friend...and then have him take your computer for a dive? Immediately after he concludes the dive, re-take possession of your computer and do your own dive while wearing both computers. You'll use his computer to track your real nitrogen-loading, and your computer just to see what deco mode looks like. In this way, you get the benefit of learning how to operate your computer in deco mode...without any increased risk of DCS.
As suggested by others, you could also do this experiment safely by diving nitrox while setting your computer for air or hanging your computer off a line from a dive boat. It just takes planning and some common sense.
I agree that some dive computer manuals are quite unclear. It's a shame really...and potentially dangerous.
Good luck.
[Edited later: After writing this, I think that this thread should probably be moved out of the "Technical Diving Specialties" forum. Perhaps it belongs in the Advanced forum instead.]
