Perhaps it's the mutation of 'add on packs' into 'downloadable content' that irritates me. Or rather it's the 'microtransaction' sales model. I had no problem with an addon that cost £10-15 when they were decent sizes. The addons for C&C or Red Alert or Starcraft were £15. They added essentially half the game on again, for half the cost of the game. That's fair. I objected to recent addons that are £25 but added less than a fifth of content to the game. But the amount of actual game that most 'DLC' adds is minute in comparison to the total game size, but usually doesn't cost an appropriately small amount.
There are benefits and losses, as with most changes in this world. Fallout 3's add-ons would have been cooler if they were larger, true expansions, but then we would have only got one or two of them. DLC allows them to do more ideas, though smaller in size they are not in scope.
As for their worth, it depends on how you look at it. I paid $50 for all the Fallout 3 DLC and I would say it is altogether about the size of a couple expansions, so it works out roughly on par. Point Lookout is almost an expansion on its own, and I spent over 20 hours doing that DLC with one character. Anchorage was like 4 hours, so they vary... altohether though I bet they are over 50 hours, which for $50 is a good bit of content.
Dragon Age only has one actual DLC now, the Warden's Keep. I haven't done it yet, so I don't know how long it is or how worth $7 it is, but I can report back.
In short: some DLC is very much worth the money, and some may not be. You can say the same for expansion packs back in the day though, can't you? I remember being very dissapointed by Tribunal, the first expansion to Morrowind.
It boils down to me disliking not having a physical product, and being reliant on the devs/pubs goodwill to keep their 'DLC' available
These things bother me at times as well, but it's almost too late to go back now. The rapid adoption of high-speed internet has changed the industry, and it will all be digital delivery and account-based services eventually.
(there is a time limit getting the DLC for Dragon Age on the back of the box in the UK - 30th April 2010 - does this mean that after that date it's not available any more? Or that after that date, despite being told that that DLC is free, that it costs money? If I uninstall DAO and reinstall, how much of a mess is getting the DLC again?)
I'm pretty sure that is just a time limit on how long you can register your key for the free DLC, rather than a statement of availability. I would bet they want people who wait and buy the game for $20 two years from now to pay for all the DLC, that's all.
As for availability, I bet Dragon Age is around for a long time, and the account is tied to Bioware and EA, so as long as they are around your content should be. For the future, long past that... I don't know. Somone asked on the official forum and Bioware responded that the content will work offline, so I imagine you just make a backup and install it manually some day if the servers are offline.