From security point of view, a system should prevent SQL injection, cross-site scripting and other potential vulnerabilities. But from a developer / architect point of view, the approaches to prevent them are quite different.
One might say that as long as we can stop any SQL injection and XSS strings from entering into a system (exhaustive prevention method?), we win. I'd prefer defer the prevention until as late as possible, just before the code is about to execute. That said, the architect of a system shouldn't try to list all the ways information goes into a system, and predict what could be the new ways in the future.
For SQL injection prevention, the only check point should be in DAO layer. For XSS prevention, the only check point should be in Web presentation layer. These are the ultimate solutions.
Yes, you can check every user input for SQL injection, but what about all the inbound messages and documents from messaging system, all the responses from 3rd party web services? As long as the information won't go into database, drop table is not a problem at all. Likewise what's the point to prevent script alert('attacked') /script from being printed on a printer?
On the contrary, you check all the input for XSS, but by accident you DBA (who knows every detail about SQL injection) inserts script alert('attacked') /script into database and this field will be presented in browsers. Can your system handle this?
That being said, dump the exhaustive prevention method. drop table is not a threaten as long as it never hits your database; script alert('attacked') /script isn't either as long as it's never presented in a script runtime.