I only feed the birds if the weather gets so harsh they cannot easily forage. Otherwise they have plenty of things in the garden they can use to eat or nest in. I agree that the birds (and toads) are very handy for keeping the bug population down. I like the snakes to keep mice out. I think it is better to be eco balanced then just offer up hotel space. Feeders leave seeds and husks that not only start the weeds but also attract other things like ants and bugs.
In all fairness though- some of the song birds and more endangered birds can benefit from some additional care. Fruit, berries, maybe some peanut butter all help, the best thing you can do to help is make sure there is a water source, even in the winter. They have heaters to prevent the baths from freezing. Well worth the investment.