It's not about getting things for free, it's about getting a complete package.
If everything is sold separately, there's no common standard in the online community: two years down the line, there's people with all sorts of different flyables that can't fly some online mission because they don't have the necessary aircraft installed.
I'd much rather pay $50 for a 10 aircraft expansion, even if i only use half of them on a regular basis, rather than have the same total price (ie, $5 per plane, 5x10=50) but with the planes sold individually. If i can buy them separately, i'll most probably buy only the 2-3 that i use the most and so will everyone else, which is what causes the multiplayer discrepancies in the long run.
That's what made IL2 successful in multiplayer, most people had similar content in their games and everyone could join everything. Even with mods, there's some standards and versions.
But when a game is cut down in small little pieces you lose all that. Imagine two years from now where the CoD series will have maybe 50 flyables in total, you join a server only to find out that every second or third mission is using aircraft that you haven't installed and so you have nothing to fly, even if you can still see them flown by the AI and other players. Then you have to wait it out or disconnect, join another server, rinse and repeat.
It's cumbersome, annoying and almost classifies as sabotage to the success of multiplayer in the long run. There's a reason that there's more people flying IL2 multiplayer than other newer sims with itemized aircraft installations (from combat ones like RoF, to civilian ones like FSX): if we don't have the same content, we can't really fly in the same scenarios with ease.
Another reason i dislike the piece by piece model is that it forces developers to only focus on flyable stuff, usually starting with the popular hot-rods.
If this had happened with IL2 we wouldn't have all those wonderful "crap" planes today, the Gladiators, Avias and Fokkers, neither would we have so many different ground objects and ground units to select from. All we would have is 20 versions for each popular fighter and pretty much nothing of the rest, compared with what we have now.
That's why a full expansion is always better: it allows the developer to sell the "supporting cast" (ground units, static objects, etc) that bring life to our ingame world along with the flyables that everyone seems to focus on. If on the other hand all they sell is single aircraft, then that's all they will focus on
