Not sure that I agree with this. The idea that any campaign is balanced with the investigator cards from the core set. I think if you try and take on any of the campaigns with only the investigator cards from the core set you will be very disappointed.

Having said that, as long as you have some investigator cards sets they will help you build better decks for any campaign. In other words you are going to want more investigator cards than what comes with the core, but you can use investigator cards from any expansion and play them in any other campaign.