Sure they can claim it, but it'll be hard for people reading to disregard all of the testimonials. Thousands of e-cigarettes users saying "It works to keep me from smoking" will be hard for people to ignore and will make the antis look disingenuous. That's why the more the better. Plus it shows that most people are using them as alternatives and not for nicotine cessation. And people who come across it can learn more about e-cigarettes. Also, we can make claims vendors can't on their sites. And for smokers who may be considering them, this may help them get off the fence.
There's a lot of different scenarios where a page like this will benefit - countering the anti claims that there is "no proof" is just one of them. To the average person, a million people saying "They work for me and I haven't had any negative health effects - just breathing better and feeling better" is proof enough, regardless of what the antis say.