Recognizing different birds from the same species

For a while I've been wondering about he necessity of birds who have banded legs and if that is the best way to keep track. For example, I'm under the impression that it might be possible to stay in the loop of what they're doing with or without the band around the "bird-ankle". Take for instance this robin I've spotted for the past few days, I'm sure it's the same robin even though it doesn't have a band. Here is a list of questions in regards to the band:

1. How much of a hurry are scientist in so that they must keep bands on birds. a. I can see how the quandary of "oh it is so much more efficient to keep the band on a leg, because this way we can keep track so much easier".

2. Is the reason why you study birds so that you are in control of the birds vis a vis the bird band? For instance- It seems that if you had the utmost respect for the birds one would be able to keep track with or without the band.

3. When keeping track of migrants I can see how it could happen where different birds fall by the wayside and maybe it can be difficult to identify deceased birds.

4. Is there such a thing as over zealous study in regards to the scientific exploration of animals, plants, i.e. the natural world as a naturalist?