Like most things, these can be done poorly or well. I've heard some talk about potentialy breaking your finger. What ever.
The main thing is a poorly done subhilt can be uncomfortable. And if your primary grip is the reverse grip, I don't reccomend them.
But if you mostly use a forward grip, and you have a well made example, they can work real well. Definately aid in retention.
As you guys know by know, I do a lot of primitive hunting and dispatch some wily critters with a knife. I've designed a few knives, haven't had them made yet, that incorporated a subhilt, substantial pommel, and lanyard hole as well as chechered grip to provide the ultimate in security.
Again, I work and fight mostly from a forward grip, and most people aren't wrestling with aligators hanging halfway off a 15ft jonboat with a harpoon with several yards of line hanging from it, while trying to slip the knife between scutes and into the neck of a prehistoric monster in order to sever it's spinal cord and end the fight. So as you can see, it's kinda a niche thing.
A subhilt can be a nicety, but not a neccesity, so I can kinda see where Desert Rat is coming from. More often than not these knives sell for looks. Not always, and it doesn't mean that you bought one just for looks, it's just that the same people who are buying stupid knives just for looks are also buying these just fot looks. Unfortunately, there are a lot of people buying for looks.
The again, I guess it's their right, even if I disagree with them and find them real annoying when they try to convince me something's real great, even if it isn't, just `cause they think it looks neat.