I wouldn't say roles have been abandoned so much as loosened-- a Slayer Fighter is a Striker, while a Guardian Fighter is a Defender. The Cleric of Pelor is a Leader, while the Cleric of Moradin is a Leader/Defender. (Almost a Paladin.) The Rogue is a Striker and the Wizard is a Controller.
So it isn't that roles are no longer prevalent, but that the relationship between class and role has changed so that a given class can choose between different roles or straddle two roles with less complication and potential dilution than 4e Hybrid.
I think this might be a little off. It seems to me that the themes don't really define your role, so much as define how you pull it off. (I think Mearls or someone said that a class defines WHAT you do and a theme HOW you do it.)
The "slayer" theme, for example, does little (that we know of, so far) to aid the type of "focused fire" that strikers specialize in. A rogue or archer-ranger, for example, would seem to have little use for this theme. (When a rogue has more than one enemy in melee range and/or is missing his target, he's not having a very good day.) However, a defender - who is supposed to be surrounded by enemies and may care about consistent damage more than high damage - would find this theme a good way to do his job while also dishing out some extra damage. (The "guardian" theme would help him "defend" in a more direct way.) Meanwhile, the "lurker" theme is pretty much useless for a fighter, and the "magic-user" theme, while it would be cool and even useful for a fighter, wouldn't really do much to make him a controller. He would be a defender with a few controller-ish abilities.
So in other words, a fighter is always a defender and a cleric is always a leader, etc., at least in my interpretation; the themes and possibly backgrounds allow you to "hybridize" to a degree, but probably not to such a degree that they seriously alter your role. Moreover, from what we've seen, the themes with the greatest synergy for a given class play into that class's primary role.
Now, all this could well change for classes like druid and monk that are somewhat hybrid in concept.