That's exactly what we do in LedgerSMB. More to the point, the db user credentials are application login credentials so the application has the db permissions of the user. No password? The app can't access any data.

Now this has some significant tradeoffs. It is very much a good decision for an ERP app. However for customer portals we are looking at a different approach, namely:

1. A portal user account in the db with very restricted permissions (and no direct access to tables)

2. A portal back-end user account in the db with limited access to tables and which owns most of the security definer functions the portal uses. I see no reason why we might need dynamic SQL in such functions, so no avenues for SQL injection unless you can exploit a trigger or something. For fine-grained permissions, the functions could do additional permission checks based on an auth token.

This should effectively prevent SQL Injection from being particularly useful in this context.