Absolutely HATE HATE HATE this change. It takes up space unnecessarily, looks terrible and is distracting. Spent a while trying to put it back to how it was before before I googled it and realised with a shock that there was no option to reverse the change. Well that discovery was extremely disappointing and absolutely wrecked my whole morning.
Particularities of the email userface should be something that is controlled by the individual user. Especially a change like this that actually reduces workable space on the screen.
This change is not practical nor functional. It has obviously been made by some employee at Microsoft in order to justify their existence at work. Please explain to this person that there are better ways to make an impact at work than aggravating a large proportion of your customers. Then to illustrate how this can be done, tell them to fix it so that this change is optional rather than mandatory.