Ingame ads would most likely come with mandatory online play to keep the ads up to date. And they'd have to use different ones for the different geograpical regions (no sense in advertising something to a player in a country where something isn't even shipped to or where it is much cheaper to buy the local equivalent instead of ordering in the US), what would further complicate the matter.
What I'm okay with are the product placement type of advertisments like the aformentioned Ikea, H&M and the infamous Katy Perry pack. The items go into the catalogue with players retaining full control over what they want to place in there games and when.
