Why on earth would anyone waste an inventory slot on something that merely docks your ship a lot slower?
If the docking computer were to be a viable option, it would have to speed the process up, not slow it massively like is the case now.
Personal opinion below - I've never used the docking computer seriously for an extended period of time until I got my Anaconda. It used to be really bad, then it got better but basically broken with outposts still (pad hovering) and since I had a Python, that wasn't acceptable.
Anaconda, however, doesn't suffer from outpostitis since they can't land on those, and since it's such a slow ship in general (and expensive, making me careful), landing isn't that much slower as long as I line it up well before engaging (sounds like 1.3 will fix the slow approach which is awesome).
The main reason, however, is because Anaconda is annoying to land. The "hood" is huge and blocks most of my view. As a comparision, the T9 which is also a huge ship, is super easy to land since, with headlook, you can basically look right below your nose.
In addition, I enjoy the break it gives me.