I think it is a great idea.There are several similar efforts, but merging them together is the problem.
For example: RootsWeb WorldConnect offers a place for people to post their information, and then they make is searchable for others.
GenCircles is another relative new effort with a few more bells and whistles.
These two sites are free and they swear they will not republish or sell your data.Many others like Family TreeMaker and Ancestry.com offer similar services, but they take your information place it on CD and sell it.
The LDS site does the same thing, they don't charge much more than the cost of publishing for their CDs.
All in all, there must be a dozen or more with similar offerings.But, you pretty much have to find the info and merge it with yours.
If you have a specific site that you are thinking of as an example let me know.I'll post some links below in case you have not seen all these sites.