The bananas themselves won't really help much because, although, like the skins, they are rich in potassium, they are really difficult to process. The skins dry in a domestic oven beautifully crisp and break up very easily to be mixed with magnesium sulphate (epsom salt) for a more 'rounded' fertilizer. The skins help with flowering in both pots and the garden, the rose garden particularly, but should really be used with other things for all round feeding.

Best on the compost heap, banana skins, if you're not going to use them as Sarraceniac describes. That goes for the whole banana too, though it seems a waste not to eat the fruit...

I usually shred the skins and throw them around my roses and clematis, but occasionally do same with whole bananas which have gone 'past it' - OH buys too many at once. It all seems to break down very quickly and my roses and clems have been superb since began doing this a couple of years ago. I have wondered same as you, i.e. whether they are good for garden generally.