7 channel avr's put less and less power out as more channels are driven. 110w x 7 may put out 2x140w, 5x110w, 7x100w, for example. Some lower end avr's are noticeably worse, and may truly be limited by the power supply such that biamping really doesn't provide any extra power at all due to this behavior.
Another reason it isn't plainly noticeable in many cases is just a simple by product of how power aligns with sound levels... 2x as much power is required to get 3db increase, which is the threshold of notice for most people. Not to mention most power is taken by bass, and since you are not raising the power output of any channel, bass spikes are still going to be the limiting factor before clipping.
Add those factors together and and it should make sense that bi-amping from the same AVR commonly results in a very small benefit, if any. Now if the technology built into avr's were to advance to the point that the front two low end channels could be given priority, and active crossovers were built into the high and low bi-amp channels, then there would be a much more favorable situation. But this is not the case except in very rare AVR's.
Panasonic had a digital model (maybe several, I have a XR-70? in my workshop) that would actually use 6 total amps (2 lf / 1 hf per side) for stereo bi-amping. In that case the difference is fairly obvious.
Having said that, all it costs is an extra set of wires, and if you routinely listen at very high volume levels, there is no reason not to do it. That extra 1-2 db of headroom may just take the edge off.
On a different note, Bi-wiring, imho, is a complete waste of time. Though there is some merit in making speaker wire jumpers instead of the normal jumper plates that come with most biamp capable speakers.