Secondary active transport uses electrochemical gradients as a source of energy for the uphill transport of substrates (coupled to downhill transport of the ion).
However except for in a few cases (e.g. Electron transport chains) the electrochemical gradient is formed by ATPase pumps which use energy from ATP to pump the ions.
So why not just use ATP in the first place (and not have to set up the gradient). (I do understand that the gradients are useful for signalling, action potentials etc. but that is a seperate issue.) i.e. What is the advantage of secondary transport.
My (complete) speculation: Is secondary transport perhaps used because it allows a sort of inherent regulation. Stop the gradient forming pump and you immediately stop all the secondary transport using that particular gradient? If that is the case are secondary transport proteins in groups with similar functions/pathways utilising the same gradient?