Your extra person does take an extra room, that is the point you are missing. You can't just cram them into existing resorts that are set up to hold only 4, fire codes and other things will not allow it. Disney is not set up to just charge you for an extra person, cause they don't charge by the person, they charge by the room.
Yes resort choices for familes of 6 are limited, cause families of 6 are not the norm! I like the car comparison. Would you complain to Chevy that their SUVs cost double of the cars, and they only hold 2 more people? Same thing.
Okay, sticking with the car comparison, but taking it further, companies do make minivans with price points that are in between a car and an SUV, with various levels to appeal to the budget conscience family and the one who wants all of the bells and whistles (Nav system, DVD player, leather seats, etc.). You do not have nearly the same options at Disney.
You are right that Disney charges by the room. The point many have made here is that Disney should offer more room options for larger families than they do currently. In terms of Disney having the space...well Disney renovates all the time (see AKL villas put in on Concierge rooms at Jambo). It comes down to a Cost Benefit Analysis done by Disney.
Ultimately Disney will do whatever it thinks will make them the most money. One of the reasons I am surprised that Disney does not offer more options for larger families is because of the minimal marginal cost compared to the substantial marginal revenue per larger family. Someone used the term Step Function Costing (I think it was Bicker). That's exactly right. The rooms are fixed costs. The marginal increase in cost per room used is minimal (electricity, water, transportations for additional guests, etc.). However, the marginal revenue per additional person visiting Disney is huge (park tickets, dining, merchandise, shows, etc.).
The latest buy 4 nights/get 3 free deal is a perfect example of this. Disney is simply trying to get more people to the parks for a longer period without any explicit additional cost to the consumer, precisely because Disney knows that the increased revenue will be significantly greater than the cost involved.
Another reason accommodating larger families makes sense is that they provide additional value for Disney, more so than singles, couples or smaller families since there are more people, which translates to increased revenue. Additionally, to get 6 people to visit Disney, say three different couples would need to be targeted and marketed to in order to get them to attend. With a family of 6, it's just the husband and wife. The downside is, perhaps, less discretionary income to spend. I dont know how many families of 6 are dinning at Victoria & Alberts!
In the end, I dont think anyone here was seriously complaining or is mad or bothered too much. Just like every other person, regardless of family size, we all make sacrifices in certain areas so that we can afford luxury items like a trip to Disney. What has been expressed here is larger families simply trying to maximize their dollars, and noting that for a family friendly place, it isnt as easy as we might have thought it could and should be.