It depends.

Is your app CPU-bound? If so, then DCI probably isn't a great idea for you. In fact, building it in Ruby may not be the best idea.

But if your app is IO-bound (like most Rails apps), huge and it takes new developers weeks to get up to speed, then the gains from having a simple, modular code-base should save you developer-time (and hence salaries), which are much more expensive than CPUs.

Having said that, I tend to use SimpleDelegator instead of dynamically injecting stuff into objects.

EDIT: added 'ruby not being the best idea if CPU bound'