Not necessarily my view on the subject, actually I have no views on this at the moment, but, you asked and seeing how I research for a living, here are some answers.
"Banding allows the determination of the minimum length of time that an individual bird lives.
Without an individual marker, there would be no way to determine if the Cardinal that is outside your window is the same bird that you saw last year or not. With a bird band, if you catch that Cardinal today and band it, you will know if that one bird is caught again in the future.
It has been learned, for example, that it is not uncommon for individuals of some species to live 10 to 20 years or more in the wild. Small songbirds that we may think of as short-lived may live
a surprising length of time. There is a record of a hummingbird living as long as 12 years!
However, the average life span of the majority of the individuals is much shorter.
Banding and marking birds can also be used to estimate the numbers of birds in a population using a mark-recapture technique. Birds are marked in one time period, and then recaptured or resighted in a later time period. The number of birds marked in the first period and the ratio of marked to unmarked birds in the population in the second period allow the total population of birds to be estimated.
Banding data allows for the comparison of normal, wild banded birds with birds that may have had their survival altered by exposure to oil or other hazards. Survival and Productivity can be
studied by using a constant effort banding site.
Birds can be vectors of diseases which effect people, including encephalitis. Lyme disease and now West Nile disease. Sampling wild birds for serious disease helps determine the prevalence of the disease in the population. Banding allows for birds that have been sampled once to be avoided in the next sample--or to be resampled, depending on the study".