I prefer paid for expansions to monthly fee approaches. Like guild wars versus most other mmo's. You can take or leave any of the expansions as you see fit, adn the expansions are humongous! And no monthly fee. If I haven't played in 6 months and suddenly get the urge to play for a day and just a day w/out paying any money, I can. There are still free patches and content updates too, but the real big stuff -> paid for. Plus, I can just leave it on my HDD, unplayed, just waiting for the day I'm in the mood again, w/out feeling like i"m wasting any money. Thus, even though I've played some other MMO's off and on, for a month or two, only GW has kept a permanent place on my HDD, and I got every expansion.
Of course, it also comes down to - what exactly is being offered in the expansion. Is it worth money and is it worth what's being asked. That's up to the individual. And I'm sure, over time, smaller little additions would be grouped into relatively cheap bundles for those that didn't like the initial price, but did like the idea.
I also like free additions, of course. Look at oblivion/morrowind/unreal tournament 99, games w/great editors allowing practically anything, literally thousands of user made mods, total conversions even - all free. Didn't stop me from paying money for shivering isles, the 2 morrowind expansions, all even in spite of all the free mods I had and ran. Get the best of both worlds that way. Man, the hours I put into those games. On the other hand, charging 1.99 for a horse armour skin when oblivion first came out, I mean come on, ridiculous. But again, the decisions are then up to the buyer. Stardock deserves to be paid for their continuing work, and the customer deserves to decide whether or not they think its worth what's being asked.
On the other hand, expansions that are little more than bug fixes should never be charged for. Bug fixes, and that includes new additions that are being added as workarounds to some problem or other, should always be free; just giving the customer what they originally paid for after all.