That's just my opinion, but there you have it.
One of the PRIMARY uses for a light on a night dive is for SIGNALLING.
For example, you can't exchange "ok" with your buddy easily on a night dive. So what you do is make a circle on the bottom (or in the water column) to ask "ok", and he makes one around your beam "spot" to reply "ok".
Another example; "heh, I need your attention" is a quick waving of the light back and forth in a plane where your buddy will see it. He will see the "flash", even if he's not quite sure what's up - he knows to look. You can then use your lights to illuminate your hand and use your normal signals - once he knows where to look!
Since you are (almost) always looking where the light is pointing, this means he can figure out where your EYES are any time he wants to, AND he can tell you where HIS eyes are pointing if there's something you need to see.
That's IMPORTANT.
Finally, one of the CARDINAL rules of a night dive is that you NEVER "flash" your buddy's face. You do that, you ruin his vision for a couple of minutes and he's going to be cursing at you (kinda like being flashed with high beams on the road, but WAY worse - trust me.) So you MUST have independant aiming from where you're looking, or you can't look at him!
A head-mounted light would make that kind of signalling and procedure almost impossible.
A canister light typically has what's called a "goodman handle", which is a piece of metal that wraps around your palm with the lighthead attached to it. It more-or-less leaves your fingers free - kinda - you can still pick something up with it on your hand since it goes across the palm rather than being held in the fingers. The cord comes off there and then to the canister, which holds the battery and switch. Canister lights are nice, and for long exposures or really BRIGHT lights (like for caves) are almost a necessity, but for typical OW diving they're both overkill and expensive.
The way I rig my primary light (Light Cannon HID) it is on a coiled heavy-duty cord with a bolt snap on the end to my top left D-ring. The coiled cord can be "snapped together" to hold the light to my chest, or unclipped to allow me to have it in either hand and extend my arm and point it as required, without the risk of losing it if I drop it somehow. I take it often on daytime dives when I'm hunting, because sometimes I want to be able to see inside a wreck or under a keel or ledge to see what might be in/under there to shoot or (in the case of lobsters) grab.
BTW, make sure your attachment is good. I made the mistake of trusting the plastic "snap hooks" that the coiled cord came with, and it cost me a Sunlight C8. Jumped in the water, swam down, went to grab for it to turn it on and it wasn't there. Grrrrr.... Needless to say the attachment hardware got swapped out for a bolt snap on the BC end and a SS split ring on the light end before I put it on the light cannon.