I find it to be a very interesting subclass design decision to go with what you do with the power you are given, as opposed to who it is that is giving it to you. It actually solves a problem of player expectation and helps save space in the first PH.
As it stands... the three "pacts" that Mike illustrated are indeed the three methodologies that warlocks have exhibited thus far in the game-- the melee fighter, the caster, and the binder. This stands in opposition of the much larger number of patrons that have been created in the game thus far-- the fey, the infernals, the star, the dark, the vestiges etc.
If WotC created the subclass mechanics off of the patrons themselves... there's a good chance they wouldn't be able to include ALL the patrons as options in the first player's handbook because there's just too many of them. Some would have to have been held back due to space. And that would cause consternation in a good swathe of the playerbase-- knowing that they wouldn't be able to play their Dark pact warlock from the beginning of Next for example.
But if instead they design it such that the choice of patron gives much more of a fluff and story overlay to the character, as opposed to a list of specific abilities that need to be described for levels 6, 10, 14 etc. etc... you can include many more patron descriptions in the first book of the game. You can have a paragraph written of each of the six to eight patron types, without needing a long list of game mechanics for each of those six to eight. Instead... you only need to list and explain the game mechanics for three-- the blade, the book, and the chain. As you create your character, you select a patron as the background and storyboard for who your warlock is... and then you select your pact to give you the mechanics to show off what power you are given and how you exhibit it.
And to top it off... it also means that every type of patron can have warlocks of all three types of subclasses. You can have fey blades *and* fey casters, *and* fey binders (as well as infernal blades, casters and binders, star blades, casters and binders etc.) That will give many more options to players for deciding how they want to exhibit the abilities their patron has given them.
I think these three pacts-- blade, book, and chain... will actually give more options for people right off the bat, and also make the creation of new patrons easier to accomplish (since you won't have to create a whole list of game mechanics to go with them.)