Well, tax credits are there for different reasons. Some tax credits are there to impart progressivity to the system, such as personal tax credits, disability tax credits, seniors tax credits, and so on. Other tax credits are there to try to encourage people to behave in certain types of ways—giving to charity, giving political donations, riding public transit, putting your kids into fitness programs, and so on.
My concern with these tax credits is that they're not very effective, for one thing. In many cases they're just credits that go to people who would have done the same thing anyway. Public transit is a good example. Most people who get public transit are not changing their behaviour in the least possible way.
Moreover, the credits tend to be almost regressive. They tend to go to people in the middle class. The educational savings tax credit is a good example of that. In some sense the RESP is a good program, but it's not very well targeted.