Light boots can be softer flexing which can make it harder to ski well. There is a good thread about stiff boots in the Ask The Boot Guys forum. http://forums.epicski.com/showthread.php?t=58878
Among telemarkers, as skill level increases, lighter, less supportive boots can be used and often are. Telemarkers like ankle flex though, while there is some debate among alpine skiers concerning the desirability of ankle flex.
Phil, is Raichle still making those? Dynafit makes light boots. Here's a boot from Scarpa that's lighter than Flexons. http://www.wildsnow.com/articles/scarpa_f1_boot/f1_review.html
From what I've learned from another thread "plug boots" are lighter than off-the-shelf models and are very stiff and high performance.