A game should be able stand up on its own, THEN you add addons, maps, expansions etc...
The way you guys want it, is that people who don't buy the addons, don't get to experience all of what they payed for, meaning if people aren't satisfied with what originally bought, A VERY LIKELY chance is that they WILL NOT buy any addons.
Also, cater to the majority, it's common buisness logic, because in the end who's gonna make you more money?
The millions of people buying the 60$ disc, or the possible million people buying 10$ map packs?
Dunno about you but I can guarantee its the first one...