While I love moddability and all, it only really seems important to me if the modding community would be doing a better job than the developers. For one, I actually don't want DayZ, so that doesn't sell me on ARMA II. However, Minecraft having such a gigantic mod community, and such slow development, I have to download Minecraft mods to layer on the complexity I need to the experience. Likewise, Unreal Tournament 3 ended up having a comparatively crap modding community, so I ended up playing it far less. Particularly because its gameplay ended up being vastly more shallow compared to Unreal Tournament 2004, which actually had a wonderful mod community.
However, the fellows at Arcen are anything but slow in their development. A Valley Without Wind alone got two major updates within, what, a month and a half or less? Patches are more 'major overhauls' with how much content they add. I doubt these games ever need mods, aside from maybe Tidalis. But, that's just me. I wouldn't use a mod, just because the regular updates more than cut it for me.