I guess google doesn't make things 'non' beta till it becomes famous beta means - it still doesn't work. And I don't think it does.
You got me there buddy I think gmail being beta has more to do with maturity of AJAX than anything else It still crashes my safari many times.
Because products are always a work in progress. Thats why one will see such things as "new and improved" on products that are like tide, pepsi, coke, nike et cetera. A work in progress, how many things is Gmail updating and improving ever few months. Thats why, its not a completed version or a final product as of yet,
Beta just means they can take it away if the want. At the end of the day if you're getting traffic from Base/Product that converts on a regular basis, you should have little worry about the 'beta' label.