Having just played through Mass Effect 2 and 3, I couldn't agree more with you. The singleplayer `DLCs' felt like bits of the game they'd deliberately chopped out so they could sell them to me later. Nothing in those add-ons was essential, but it all felt like stuff that would have been included in the game proper ten years ago.
Of course it turns out that on this occasion I picked the wrong target as they are doing the DLC thing right (they started work on it after the main game was complete rather than it being cut out of the main game and sitting on the disk from day one ready to be activated by a DLC code) but that wasn't made obvious in the sales blurb I saw at the time.
But my principal still stands: if the DLC is genuinely an extra or an extension then I don't mind, but when it seems to be a core part of the content removed to get a few extra $ I'l adjust what I'm willing to pay for the main game accordingly.