If it is paying 7% tax free with no loads why isn't everyone just pouring everything in.
That wouldn't be the way to go for many reasons. It would pose some very serious risks to dump all of your assets into one section of the market. Diversification is crucial.
Here is a link to an excellent discussion about expected future returns:
But check out Bernstein's book. Actually, there may be some others you should read first to get a feel for the subject. His book is the best I've read, but I don't think you'd want it to be the first - you might try Personal Finance for Dummies by Eric Tyson. The title is goofy, but the book is excellent and won't lead you astray (many will). Bernsteins book is excellent, but there's an assumption you already know the basics, IMO.