On the tactical side, freq hopping on VHF is WAY easier to un-ass than Havequick on UHF. And if you want to talk to a helo on UHF you're screwed unless they're on top of you.
This could spark quite the interesting discussion... First, I've almost never tried to talk to a ground unit via Havequick I/II. Single channel secure or SINGARS is the only thing I've ever used. I have used Havequick to talk interflight, and in our aircraft it's the opposite. Havequick is way easier than SINGARS. Unless there were good SOPs in place, talking SINGARS was painful at best (because our time is constantly downloaded from the GPS), because if the ground unit doesn't use a PLGR enough, we can't talk. As I'm sure you well know, the PRC-119 is notorious for losing time.
Of course, talking to a helo on UHF has its limitations, but it's all a factor of the radio/power. I'm assuming your reference to screwed is using a manpack radio. The PRC-148 is pathetic, not enough power without the amp. The PRC-113 is better (more power), and the PRC-117 is best for a manpack. Still have that pesky line-of-sight issue. Using the MRC-148 is a whole different ball game, I had no issues - could never see the aircraft, and could talk to them sometimes close to 15 miles away. Throw in the fact that I didn't have to walk anywhere, had UHF/VHF/HQ/SINGARS/SATCOM/HF, and an ass ton of power, and I was a happy man...
IMO, VHF enroute adds little to my SA - Busy terminal area is where I tend to use it.
This was my rule of thumb for VHF/UHF:
Military Airfield/Terminal Area (other than Army): UHF
Army Airfield/Termina Area: VHF
Civilian Airfield/Terminal Area: VHF
Enroute when I'm bored: VHF
Enroute when I'm not bored: UHF
Why VHF when I'm bored? Because you occasionally hear some gems on the radio. Heard this one time while monitoring Center on a cross country:
"Center, Cherokee XX - ummm... I forgot my map, can you tell me where I am?"
"Cherokee XX, Center - I can tell you where you are, but if you don't have a map how can you find where you're going?"