In this case it makes sense because these people already do consulting for a living, so this is just another way to get clients. And further, the person calling is going to be looking for expertise about a specific topic rather than looking to talk with a specific individual because of their accomplishments or whatever, so it's much less socially awkward.
I do still think there is an opportunity though to get celebrities who wouldn't normally do this to do it, it's just that none of the ways that have been tried so far have been suitable.
It's not just consultants. They connect you with industry experts. One of the biggest spammers on linkedin I find are these types of companies looking for domain experts.
Yes and no. I've been used as a GLG expert a number of times. The only people who seemed to use their service (read willing to pony up for the very high per hour rates) were institutional finance research groups doing due diligence for M&A or similar transactions. That's a pretty specific market with very deep pockets.
http://www.glgresearch.com/