Maybe I'm being a bit thick, but the way I'm thinking of it is this:

Flying near the pad, it doesn't really mean anything much to me what power I need to fly S&L at BROC speed, but what I'd really like to know is what power I will hover IGE (or OGE for that matter, preferably both) with.
Then I can either use a predicted or allowable maximum power, or even better, pull to one of the limits and see what's available, and apply whatever margins I want to allow for approach/departures.

eg. hypothetical helicopter - say it's TOT limited in the conditions I want to land at, arrive in the area, pull to the limit and check how much Tq available. Fly at my magic speed that equates to IGE (or OGE if I have a speed for that) hover power, then I can say - OK, can pull 97% Tq at the TOT limit, I will need 88% to hover IGE, got a 9% margin which I know is heaps for an angled approach to the IGE hover. Or something along those lines.

Flying at BROC speed doesn't really help with what's going to happen in the hover - or does it??

Arm out the window,

The speed is not critical for this. What you are doing is applying a rule of thumb that you have worked out previously (or someone else has) to be able to estimate the power required to hover IGE at the DA you are about to land in.

What you want to be able to say to yourself is this:

"This helicopter uses X amount more power to hover IGE in nil wind than it does to fly S&L at my reference speed at 500' above the landing DA." I used BROC as a good reference speed for the reasons previously mentioned.

An example (I've just plucked the numbers out of my head, and notice that 80 Kts is not BROC for the Jetranger):

You have worked out that your Jetranger uses 15% more Torque to hover IGE in nil wind than it does to fly S&L at 80 Kts 500' higher. So you set it up S&L at 80 Kts, 500' above your landing site, and you are using 75% Torque. You then know it will take about 90% to hover IGE. You can then compare that to your maximum power available and determine your margin at the bottom.

If you don't know a rule of thumb, what I am saying in my original post on this is that you can work one out for yourself by experimentation.

Armout - the reason for using BROC speed is that someone has bothered to go to the trouble of working out how the power margin available at that speed relates to the power required in the hover ( see Mr Selfish's post for the R22 figures) and therefore what sort of approach you can fly.

Undoubtably you could experiment with different speeds and see how the power relates to the BROC power and maybe come up with a rule of thumb to convert cruise speed power available to hover performance but since you have to come back to 50-60 kts at some stage before landing then that is the time to check your power.

Thanks guys, I just thought that there may have been some theoretical basis for using BROC in that the relationship between that and hover power was predictable, or something along those lines.
I guess next time I think of it I'll look for the relationship between hover power and power used at various speeds, and also see if it's repeatable.