Using pure functional languages can have a lot of benefits over using impure imperatives, but low level systems languages will generally allow you to achieve much greater performance, especially when they are imperative because it allows you to specify the exact steps in how the cpu should compute the result.
If there is ever list of tools where high performance is an absolute must then I would put VCS at the top of that list, and git achieves this very well. However performance is not git’s only advantage over many other other types of version control systems anyway.
The git team is handling the unsafe c code very well, and I never worry about my type system (or any other features of the language it is written in), so why is it that there is a lot of haskell developers that must use darcs when they will only be using the finished product?
I think in general certain communities of users will gravitate towards one technology or another. This is especially true if a technology uses the community’s language. It is also possible(though I have no idea) that the author and/or initial users of Darcs were fairly well known Haskell developers, thus having more influence.
There are other examples, such as Mercurial appears to be more popular in the python community while other languages appear to use Git.
Once you are using a particular piece of software it may not be worth it to switch to another even if there are advantages.
Honestly though use whatever you like best. At this point it is almost a requirement to have hg, git, and svn installed and know the basics to using them. If you are using Haskell it appears you probably need to add Darcs to this list.