This is a bit of a pet peeve, and something I've heard repeatedly to justify bad code and/or bad design. "It needs to stay consistent."
Question: How can code or design improve if it must remain consistent?
Code and design should be free to evolve within the lifespan of a project, to freely address concerns that come up with the initial approach and constantly challenge whether or not the original assumptions were really the best decision for the business. Going back through all existing functionality and re-factoring based on new technologies, patterns, or preferences of the customer can be prohibitively expensive, but that should not be an excuse not to adopt it going forward... <caveat> *If* it is what the customers wants. </caveat>
A consistent approach to an application is important, but it's a bad idea to take it so far as to make it extremely rigid. A common pitfall is with UI. For instance, stressing a design that says that all forms will have a default set of action buttons for wizard-like behaviour (Next, Prev, Cancel, Skip) is just asking for trouble when the customer wants to do something different. Pretty soon the "Cancel" button gets re-labelled and on screen X, performs action Y. Designing all forms to be dynamic, based based on some construct of a framework leading to slow, buggy, and not-so-user-friendly experiences is another example.
It's not that designs like this are inherently bad. They aren't, but they aren't guaranteed to be suitable to 100% of cases out there.