Well, the purpose of starfleet sure seemed to have changed over the years. A few examples from the TOS Tech Manual (Sure it is not canon but I think it is quite well in the spirit of TOS)
"There is established a Star Fleet as the armed peace-keeping forces of the United Federation of Planets............... It is the intent of these Articles of Federation that Star Fleet shall be used to conduct missions of scientific exploration and investigation within the treaty exploration territory whenever its services are not required in the maintenance of interplanetary peace and security"
So in TOS times they seemed to were -above all -a peace-keeping force and scienting only when nothing else was to do. I must admit I quite like the TOS Tech Manuall, obviously they were a lot more down to earth at those time. Starfleet is military first and a science force second only if nothing else is to do.
Thats what I like about trek XI, starfleet feels once again like in the TOS times, unlike TNG, DS9, VOY and ENT who were all really only TNG.
I'm Commander Shepard and this is my favorite store on the Citadel.