That's the latter part "build an open source alternative as good" that might be problematic due to the patent (then again, if it was obvious somebody should have built it BEFORE they did and thus have prior art).
But the first part, "because we like/want the functionality" still holds whether there's a patent or not. If there's no alternative, either because nobody bothered, or because the software is patented, then people will still use it, regardless if it's proprietary or not.
Photoshop, the other example I gave, also has tons of patents, proudly listed on their "About" windows.
