Maybe I missed it in the above posts but one theory that I ran across is that the cruciform hilt was never intended to protect the sword hand from the opponent's sword but to protect it from being jammed by the opponent's shield. That is not to say that it was never use to stop an opposing blade but to do that one is going to have block with the edge of the sword for the cross guard to be effective.

More complex hilts came into use as the shield was discarded with side rings that would stop an opposing blade by while blocking with the flat of the blade.

This is a very interesting topic, I myself have always wondered why complex hilts are seen so late in the evolution of swords. I practice German longsword with a bit of sword and buckler and kali silat thrown in and found side rings to be a very useful addition to the guard of a longsword. I agree with Vincent, I feel it gives me quite a substantial increase in security while taking away nothing whatsoever in terms of practicality. At least if the side rings aren't too small for my gloved thumb to fit under... ;)

Now of course a simple guard works just fine but for me, it was a no-brainer to employ additional protection to the hands. I have already broken a finger once in sparring and whatever keeps my hands safer without taking away anything regarding use and mobility is a big win in my book. One thing I noticed though is that with the additional security of side rings, my Ochs tended to get a bit sloppy, now that I didn't have to rely to perfect technique anymore to keep my fingers safe. I figure it's best to do partner exercises and technique training with a simple cross but spar with a complex hilt.

Regarding the argument that a simple cross is as good as it gets for longsword fighting, as it worked without side rings for so long... I don't buy it. As soon as complex hilt started popping up, they were added to pretty much every bladed weapon, including longswords. If a simple cross is all you need or desire on a longsword, why the huge popularity of side rings as soon as they became known? I can't believe that it was just a fashion trend.

As a sword maker who has fiddled with complex hilts, I can also tell you that it is NOT as simple as adding a few metal bars to a straight bar. The more complex the hilt gets, the more pain in the a** it is to get everything lined up and not hindering mobility. Of course I today can just arc weld the rings on, back in time they did not have that possibility, further complicating the process.