1. The game is complete without the DLC, with no plot holes / loose ends that require additional purchases to resolve, and all features that can be reasonably expected present and accounted for.
2. There are no ads / reminders during the gameplay for the DLC. Not even during loading screens. I'm fine with a DLC option in the main menu.
For F2P games, everything is fair game though. Wading through that crap is the price of playing.
IMO Dungeons of Dredmore and Jamestown are two examples of DLC done right. Minor quibble with Jamestown - IIRC you don't know the DLC is required until after buying the right to play those levels in the in-game credit (not really $$) store. But those credits are easy to come by, so no big loss.