Nothing, obviously. If you don't need the added features, then there's no point in you bothering. But for people who do want one or more of the added features, and don't mind learning a little more, then it is worth it.

But most of your points are simply hot air and obviously made up. Many people install custom software to increase battery performance, yet you think the opposite. They have done tests and paid attention to see if there are improvements - have you tested to see if you have better battery performance? I doubt it. You know, there are a lot of adjustments for the 2 cpus in the phone. The default is for some imaginary default user, but a lot of people don't fit into that default. Perhaps I don't play games, don't demand high performance, so I can load a kernel profile that will keep the cpu at a lower speed most of the time, increasing my battery life. Someone else, otoh, might not care about battery life, but they want a fast system most of the time. That, too, can be done.

And where do you come up with these "tradeoffs"? Do you really think only the Google launcher works well, and something like Nova Prime is a tradeoff? Do you really think Pure Nexus Rom is causing lag, and poor battery life? This is all just made up tradeoffs that exist only in your mind, but you are acting like they are facts; they aren't.

I have a rooted phone, a custom rom, custom kernel, custom launcher, and I'll put my phone's performance and ease of use against yours any day. I don't have any noticeable lag, camera problems, etc. Neither do most people I know in similar situations. Have you ever read the Nexus forums on XDA developers? You'll see lots of comments about "buttery-smooth performance" when using various roms or kernels. These people are not delusional.

So, after justifying your decision with made up facts, you say you are honestly curious. In case you are, you are missing things like the ability to use the power button to turn on the flash when the phone is sleeping - I find that pretty handy in the dark. You're missing things like double-tap the screen to sleep or wake the phone, like the LG G series. You're missing being able to customize the statusbar, navigation bar, power menu, etc. How about raising or lowering the default volume? Loading CPU usage profiles? If none of these interest you, then you are missing nothing. But don't think you are necessarily gaining anything by not doing it, except saving time doing something you don't want to do.