Why would anyone ever care to even do that......
If you want to run the game DRM-FREE, of course.
Since Steam-version here has more DLC than every other version - so if you have a DRM-FREE EXE version and it actually works when in the Steam-folder - you've got it going DRM-FREE! Why not give it a shot? Worst that can happen - it just won't work! Oh well.
In this case - you already paid for both DRM-FREE and Steam-versions - so, why not give it a shot? What the hell do you have to lose?
{Shrug}
I pretty much did the same "EXE-plastering" trick for Beyond Divinity from GreenMan (Capsule), just to avoid the EXE from forcing a client-program (Capsule) on me. Why? The install of BD from Retail discs includes StarForce DRM files. GreenMan's download DOES NOT include StarForce - and it's easier to deal w/ that download-version since it's all patched-up (Version 1.49); and comes as one set of files (opposed to say 2 CD's from Retail). So, I do already have a patched-up version of BD from retail, which has a DRM-FREE EXE (thanks to Patch 1.49) - so it doesn't do any StarForce checks. So, I renamed the Capsule-version's EXE; and plastered the DRM-FREE EXE in there - and voila, don't gotta deal w/ Capsule!
Oh, I know - this is all hypothetical stuff, but here we go for other reasons why you'd want to try this:
Steam goes out of business and they officially do NOT patch CEG out of their service.
(probably won't happen for a LONG time, the way Valve and Steam are going - but, you never know).
Or say someone doesn't unofficially crack the DRM.
Say offline-mode doesn't work properly for you.
Say Steam client-program doesn't work on much more modern OS's.