Why shouldn't I give outsiders access to my database?

Question

Lots of sites today have APIs that allow users to get data from the site as XML or JSON using a GET HTTP request. Flickr and del.icio.us are example of sites with APIs. These APIs require the server to access the database, and then output the result as either XML or JSON.

Why do we need this translation though? Why not just create a user on the database (for example MySQL)? The user would be given limited access to the database, only being allowed to SELECT, and only certain tables and certain columns in those tables. Wouldn't this be a lot more efficient for the server (it wouldn't have to deal with the HTTP request), and it would be easier for developers, who could now access exactly the data they need, the way they need it.

Answer 1

Security considerations aside, so that you can change your database structure without affecting your clients. Also, poorly formed queries tie up your server, not the clients.

Answer 2

Can you prevent a malicious individual from crafting a super-complex SQL query that will peg your database's CPU at 100%? Can you prevent a lot of innocent programmers from crafting inefficient queries that will never be optimized that will do the same thing?