Rooting, ROM'ing, Unlocking is developer driven and Google likes to keep an eye on what these Dev's are doing. So instead of locking it or voiding the warranty they stipulate you must be able to return the phone to stock (factory image). If not all bets are off.
It helps them understand what some users like and dislike by seeing what they enable or disable or create.
For instance changing the battery level display icon and clock position with date /Time format options. These started on custom ROMs'.
A screen brightness slider by touching the top edge of the display and wiping left ot right to get your desired brightness.
Google listens to the masses and reacts sometimes.
So as this all evolves, the more Google/Android adopts the less Rooting/ROM'ing will be needed. But for me I keep my ear to the ground on what ROM's are developed and if one looks good and is stable having a feature I can't live without I go after it.
Down sides according to the developer capability or if it cannot be tweaked are sometimes things like Android Pay will not work on early versions. All of this is explained in the ROM description and release notes of any given custom ROM.
But ROM'ing is getting less and less necessary in baby steps. A big leap would be light and dark ROM options. I understand Google experimented briefly with one.
A similar evolution example would be Harley Davidson. Years ago they seen the custom aftermarket accessories market was booming. They proceeded to tap into the market by turning out bikes already equipped with what was trendy and raising the prices.
So I go with Google for the vanilla, and want an unlockable bootloader just in case some amazing ROM pops up with stuff I need.
Since the Galaxy Nexus my Google devices were Rooted right out of the box after I made sure it worked OK.