The main problem, as others have stated, is the contractural obligations that the channel owners impose.
In my case, I care not for sports, kids programming and mainstream news so the ability to have a base programming package that doesn't send my money to ESPN, Disney and MSNBC is a plus. If I want Fox news or BBC World News I can add the News pack for $10. If we have kids staying over for a few days I can add the kids channels and get rid of them later. Last time I checked, adding and removing incurred no fees except for the actual amount of time the add-on is active.
The bad side is some channels that will not allow DISH to put them in Flex or any of the add-ons, meaning you have to upgrade to at least AT200 if you just can't live without BBC America, El Rey or the Golf channel. For me, saving $50 bucks a month is more valuable than those few channels.