My experience is probably relevant to this question ...
Until recently I used a Light Cannon on night dives, mounted on my camera rig using the ULCS ball adapter. I didn't find it to be much of a problem in terms of handling. (But mine weighed less than standard because I rigged up an adapter to use AA instead of C cells.) However it's much too bright to use as a focus light for shy things at night, as they are either gone or toasted by the time you get close enough for that macro shot!
It doesn't have to be too far forward; even without any arms the clamp on the ball joint allowed me to get the centre of the light somewhat behind the front of the camera.
For more distant objects the light cannon is a great night light. And for closeup it makes it very easy to focus (on things that don't run away). The brightness and whiteness of the light made it the envy of many other divers.
With a reasonable diffuser, and a moderately powerful flash, I didn't find hot spots to be a big problem. However without the diffuser it was a problem.
Mine did about 50 - 75 night dives (not sure exactly and I'm too lazy to check my log books) and was turned on and off quite often (but always according to the required delay) - whenever I found something shy, until I thought of the plastic cover mentioned above. It died suddenly a couple of trips ago (leaving me somewhat in the lurch). I bought a new globe (very expensive!) and that died suddenly at the start of the 2nd night dive after that. I don't use it any more. I expect there's a problem other than the globe but I can't afford to keep trying to find out.
I was always worried about turning it off and on so often. I eventually worked out that it's easy to attach a plastic disk to the front on elastic so that I could cover it and use a lesser light for focussing,without turnign off the light cannon. So that meant having 2 lights attached to the camera rig, rather than 1. Later I got a piece of black plastic and cut lots of regularly spaced small holes in it (to drastically cut down the amount of light), together with a diffuser, and this could be slipped on and off so I didn't have to use another light.
Currently I'm experimenting with using 2 lights mounted together (because I didn't have any bright lights left after the LC failed at the start of the last trip) - a wide light and a bright narrow beam. I turn off the narrow beam when I want to get close to something shy, or when I'm worried about hot spots. Actually it works pretty well and I intend to improve this system slightly and keep using this approach. It also has the advantage that if one light fails the other is already mounted on the camera rig so no fumbling around. (My real backup still lives in a pocket.)
-David
