I have many clients and systems that use Dual Monitor displays. They always complain that they would like true dual monitor support meaning that if they have dual monitors on the remote system and dual monitors on the local system they want each remote monitor to show in its own local monitor so that the experience is the same as sitting in their office with dual monitors.
I agree that this would be useful but I can see how this might be a challenge to implement from a development standpoint. Having to deal with mismatches in the number of monitors might be tough. I have 3 monitors at work but only two at home - this might be confusing to deal with gracefully.
As a suggestion I could see a very simple solution being having the ability to open a new brower window for second and third displays. It could even be achieved right now if you were able to log in 2 different sessions into the same computers. I think a simple fix though would be to give us the abilit to just launch the second monitor into another browser window from the same session giving us true dual/triple monitor support from a remote session.
Actually LogMeIn is not really running in a browser, it is a Win32 app at its core and just uses the browser to launch itself. So it should be possible to have UI and options to dictate which screen goes where. They can copy some of the UI and logic from tools like 'Input Director' and Synergy.
I bet it just requires a lot more bandwidth to send that second monitor and also some more code in their video driver hook.
I would have to agree with you on the perspective of the extra bandwidth for the second monitor, but i guess it might come down to a user experience preference, running in grayscale mode with dual monitors might be a better experience than full color mode with one. :)
Hopefully there might be some development on this avenue.