So why didn't you get yourself a Galaxy S2 or S3?
I would go as far as to say that the Touchwiz Nature UX on Jelly Bean 4.1.2 (Galaxy S2 Test ROM) is much nicer than stock Android.
Wow, where to start...
The obvious answer to your question is that I didn't know what I know now about the screen BEFORE I bought.

Not that it matters, it wouldn't change my decision in the slightest. As I said, the screen is not what I expected, but by no means "bad". In fact, on paper, and according to MOST people, it is superior to any AMOLED out there. As CrazYmonkeY159 said, it is a very subjective thing and the topic of much debate. Screen quality aside, the Nexus 4 is in my opinion better than the S3 in every respect.
However, my primary reason for moving away from Samsung (I'm coming from a Note running Jelly Bean) is that I am tired of being at their mercy with software updates. It has been six months since 4.1.2 was released, and Samsung's flagship device, the Galaxy S3 is only now getting it. There are still some regions that have not yet received the update. Worst of all, despite the fact that there has been a fully functional leaked Jelly Bean ROM available online for the Note, Samsung has yet to release it officially. Why? Because doing so would impact the sales of the Note II which they are currently punting. Seriously, I'm tired of that BS.
Then there's the issue of Samsung's refusal to work with the developer community in opening up their platform. As a result, after-market ROMs such as CyanogenMod (which is my ROM of choice) are half-baked on Exynos devices such as the S2, Note and even the S3. Its just a sub-par experience, and it shouldn't have to be.
Also, unlike you, I 've never cared for TouchWiz. I far prefer vanilla Android. Again, this is another subjective thing, but that is what I prefer.
What's the point of running a custom ROM on a Nexus device?
Rooting and modding the notification bar for quick toggles would be as far as I'd take it personally.
I am running CM10.1 on my Nexus 4. Frankly, the better question is what's the point of NOT running CyanogenMod when a solid build exists for the Nexus. CM takes everything from vanilla Android (yes, this is still the latest version 4.2.1, built from source) and throws in a bunch of enhancements and tweaks. Seriously, after a few hours with the stock Nexus 4 ROM, it became quickly apparent that there were just too many little things missing that I wasn't prepared to give up. I've used CM for about four years now, across a number of devices and on the Nexus 4, it is even sweeter than I could have imagined.