I'm sure they can be very valuable producers of useful things needed.....but...Prior to the invention of nitrogen fertilizers pigeon poop was so highly prized that fortified dovecotes were devised to prevent its theft. They also produced eggs and squabs for eating.
This link tells more about dovecotes.
http://www.milcombe.com/dovecote/
This guy did not harvest the fertilizer, nor eat squab nor pigeon eggs.
Kinda like alot/most the folks on BYC keep chickens as pets/surrogate 'children' rather than food sources.