Need? No. Want? Yes.
I mean, lookit: people bought the Expanded Psionics Handbook because they wanted psionics in their game.
The first Fourth Edition Player's Handbook is subtitled "Arcane, Divine, and Martial Heroes", and we have good indications that the second Player's Handbook will include psionics.
The only difference, as far as I can tell, is that they'll use the Player's Handbook series to introduce alternative sources of power and, presumably, alternatives to the existing core classes and races - sort of a mix of "capsystem" books like the Expanded Psionics Handbook and "splatbooks" like the Complete and Races series.
So you'll buy a single book with, say, psionics, incarnum, and pact magic, labelled Player's Handbook II, rather than the Expanded Psionics Handbook, Magic of Incarnum, and Tome of Magic.
In fact, Tome of Magic itself - presenting three different sources of magic - is probably a good example of how the Player's Handbook sequels will manifest themselves.