Sure, every pc should have a background in the sense of where they came from, how they became an adventurer, etc. What I don't like is the post-PH1 tendency to give mechanical benefits for backgrounds.
Some backgrounds are outright broken, true. Most backgrounds,though, give a simple +2 to a skill or add it to your current skill list. A rare few give a new language as an option. As a DM, if I player came to me with a fitting background, I would have done the same kind of thing anyway, as I award flavor, so I personally don't see a problem with it.
On the flip-side, if you came to me with a background that had no logical reason for your character, ie purely a mechanical one, I'd look at you quizzically and request you take something else. You can take as many backgrounds as you want, but you only get the benefit with one. I don't really see this as game breaking, but I can understand if you dislike it.
Also, on the themes. Taking a theme gives you access to one theme feature automatically. In essence, they're paragon paths for heroic level. The other available powers are only available as options. You don't get them in addition to your other powers; it's an either/or option.
I must say this about the given theme powers, they scale faster than the standard powers given. If you take a level 5 theme daily, for example, it improves at level 15, and again at level 25. (As you can see with the primary example)