They draw a salary yes, but they also produce an income for the company. This funds them AND the other team doing the other work so it isn't a drain. Let's say DLC team costs 100 to do their job. But their work makes 500. The guys working on issues like AI and other behind the scenes cost 200. So now rather than being -200, Firaxis is +200. So the DLC team are not a drain on resources, they are a positive. You see this type of process all over the world in almost every industry from police departments to airlines (ie we will green light this expenditure if you can find sources of revenue to offset some of it). Without DLC team you are FAR less likely to get a game with the development periods of today's games. Games today are patched and improved on more than ever because that cost can be justified. Remove that key profitability area and it is much harder to get money for the project. One of the key things that gets asked in any proposal is "what are your projected revenue streams" and if they aren't good enough then why would a company risk an investment? If you want to do that extra work in your proposal, you need to provide a suitable revenue stream somewhere to justify it. That is just the way business works (unfortunately).
Things like the poor UI or whatever is a totally different matter and shows that they were clearly missing key steps in their development process. Whereas AI is a much harder issue to fix in the time frames were are talking about. But there is no causation between the DLC and poorly coded AI. It's like blaming the concept artist because there wasn't enough testing of UI.