Disadvantages of using scriptlet?

You seem to concentrate on only the presentation and flow-control part of the scriptlets as in using if, for and switch statements and out.print() things.

You seem to compare scriptlets 1:1 with JSTL. This is wrong. I was not talking about the flow control part only (which is indeed to be replaced by JSTL), but about writing raw Java code in JSP files in general.

Ie gathering request parameters, validating and converting values, interacting with database and other Java classes/methods, etc. All things you normally (indirectly) do in a Servlet or Filter.

JSPs should be purely presentation. That's the hidden benefit of writing JSPs using only JSTL, because they get all their dynamic data elsewhere. Let the service layer have the business logic and determine what data the JSP needs.

This answers your unit testing question, too. You should not have to unit test JSPs; those would be Selenium-like UI tests. If the logic is in the service tier, it's obvious how you test it.

JSPs should not be inherited. You can certainly compose them together using something like SiteMesh, but inheritance has no part in your JSPs. Once they inherit from Servlet, the chain should be ended.

Besides, it's a false alternative. Neither one should require reuse, inheritance, or unit testing. But that doesn't mean there isn't a clear winner: it's JSTL. No one should be using scriptlets in JSPs, except for very rare one-liners. Scriptlets are begging for trouble.

These days I prefer Velocity as my web UI template solution for Java, much more than JSPs. Just my opinion.