I get asked this question a lot. I have heard arguments from both sides.
Personally I enjoy feeding the birds in my backyard. I love seeing the variety of species that come out and watch their antics at the feeders.
I don't feel that they rely on my feeders as their main food source, because occasionally I get busy and feeders don't get refilled for several days. This does not cause any of the birds to die from hunger (I assume I'd find bodies if they did die).
I do think they must have some great methods of communicating because the minute the feeders do get filled they all come over to sample the new goodies.
The bird food we provide is a useful source of extra energy for the birds. Particular during cold weather a little extra energy in their diet goes a long way towards helping these birds to keep warm.
It is important to remember that bird feeders need to cleaned regularly to prevent the growth of fungy and potential spread of contagious diseases from bird to bird. Water baths also need to be cleaned regularly, but in my experience most avid bird lovers like myself do these things.
An other important thing to keep in mind is that when you have bird feeders you are not just feeding birds. You might also be feeding squirrels and a whole host of nighttime visitors you are probably not even aware of. If it bothers you to have raccoons, squirrels or other critters in your backyard or if these critters are becoming a nuisance to you or the people who live around you, you need to stop feeding the birds and take down your bird feeders to give these critters a chance to move on (no food = no wildlife).
It bothers me when people put out (live)traps to trap little (nighttime) visitors such as raccoons while they have a whole host of bird feed in their backyard. It is not realistic to expect these critter to not take advantage of such an easy food source.
I also respectfully would like to ask that if coyotes are a problem in your area that you take down your bird feeders. Birds seed might not be high on a coyote's 'tasty treat list', but the critters who do eat seeds are. Coyotes are unfortunate in that they seem to stir up a great deal of fear in humans. Some of that fear is realistic (they are after all a predator), but a lot of it is not realistic at all. We are not on their 'to eat or hunt' list. The main part of their diet consists of rabbits and mice, but they will not hesitate to snag a cat if the opportunity presents it self. This is all the more reason to keep you cats indoors and to not leave your dogs outside unsupervised.
Apart from the issues mentioned above I feel everyone who enjoys feeding and looking at some of our wonderful feathered friends should be free to do so.
There a lot's of awesome resources out there. I'd like to encourage everyone to shop local for their bird feeding supplies. I can highly recommend a store such as Wildbirds Unlimted Toronto, not only has this store and it's patrons helped us raise some desperately needed funds, it is locally owned.
Another great resource and important project is Bird Studies Canada's Great Backyard Bird Count. If you are not familiar with this project I'd recommend that you check it out. The data generated with this annual study is tremendous.