There's been a discussion on this very subject on ATC Issues recently. Check out...
http://www.pprune.org/ubb/NonCGI/For...ML/000791.html
I'm certain it is difficult at the likes of LHR where you know every second is vital to us, but like Flaps One says - Cross 'em at your peril. If they are stuck on red, I'd tell you. If I've forgotton to switch them over, you tell me - I'll feel a plonker for a bit, but might not forget for the rest of the shift!
They're a safety device - why compromise them?