Well, to be honest I am not sure about that. The objection that I have seen against changing the checksum is that it would make mods or MP difficult, but all mods to HOI2 change the checksum. All people do is keep seperate installs for each mod - I have about 5 different installs of HOI floating about. If the DLC was sort of a paid mod then surely people would just keep one install for vanilla and seperate ones for different DLC "mods". With something like the mod launcher that EU3 ships with, it need not even be that complex.
I am not in favour of DLCs, but I would rather they added actual content to the game, rather than being candyfloss.
Mods change a lot of data files included in the checksum, which can incidentally get overwritten by patches on occasion if you don't use a mod manager.
With DLC adding new features the hassle can go one step further: let's imagine, for an example, one piece of DLC includes an .exe modification to allow the smooth run of battlescenarios where you play a general's career, to teach the game to give you orders and stick to them.
Next comes a patch which adresses some bugs, also fixed in the .exe. After this DLC is released, either the patch has to search for strings to modify at different adresses in the .exe file depending on the presence or absence of that piece of DLC, or it includes two versions of the .exe file and checks for what DLC you have installed to pick between them. Data files are easier, you "just" have to check for DLCs and install the relevant file.
Now let's throw a second .exe or data-modifying DLC into the mix. Your patcher has to check whether you have DLC 1
OR DLC 2
OR DLC 1&2
OR no DLCs at all before it changes the .exe or relevant data files, and include instructions/files for the four possibilities. A third DLC brings this up to 9 possibilities, and you can go on by squaring the number of DLCs after that.
Each and every time you have to include more and more complex checks (especially in case of .exe modifications), and more and more different files. It becomes patching hell.
It's no different for modders. How do we guess whether user X has DLC 1, 2& 9 which modify this data file we replace in the mod, or only DLC 2 which is really required to run the mod? Must we write 10 or more different versions of the same mod for different DLC combos?
And no, you can't make life this hellish for modders. They make a critical part of PI's customer base.
As an aside, none of the contents added by Bethesda's DLCs for Oblivion which changed important game mechanics or added new functionalities, they only minor additions like maps, textures, meshes and scripts which could have been created by modders as well (indeed, modders have far exceeded what Beth did with DLC). And Bethesda relied as much on its modding community for Oblivion as PI does for its game - modding is what made Oblivion's predecessor Morrowind such a successful SP RPG, and not hampering modders was as critical for Beth' as it is for PI.
