If memory serves, they removed the user agent because one of the Google properties was serving it substandard HTML/JS even though they use the Chromium engine.

Yes that is likely the case.

All browsers wants to use a unique user agent.

It is the only way to prove market share => $$$.

But it is ridiculously difficult to modify it without breaking sites.

Take a look at your own user agent.

It is likely it identifies itself as gecko/mozilla/chrome/safari/IE/KHTML all in one.

Think about it for a second. They removed the very most important metric for a product selling content leads and placement (search bar default bookmarks, placement on the start page) because _one_ website had a bug?

They removed it because they claimed Google was sending substandard markup on purpose.