If you look at TT equipment all team members use the same helmet regardless of the fact that helmet performance varies massively from rider to rider.
The 2016 S-works Venge is 5w faster than both the SL7 and SL8, so on flat stages all specialized sponsored teams are riding it... aren't they?
Why is nobody wearing a TT Helmet and visor on a normal road stage?
Because it doesn't work as much alone as in combination with specific aero body position. But you can't be in that position in a normal road stage because the aero handlebar extensions are forbidden in mass start stages.
Also there are less aero benefits to be had when you ride in the peloton behind other riders.
The aero bar question (whether to have them or not) comes down to how long you will spend drafting and climbing. It's a more interesting decisions in Olympics and triathlons than most road bike racing (the latter it makes no sense because people are drafting as much as possible).
https://www.cyclingweekly.com/news/racing/uci-publishes-imag...
(1) weight is not always worth it, notably when there are lots of climbs
(2) riding with others, need to be able to turn your head. Further, you can't get as super low when in a big group. It also doesn't help as much because the group dynamic is more important than crouching extra low and/or helmet drag
(3) out of the saddle sprints.
Nobody ever has…
The 30% - Drafting is good for a 30% power increase. When in the middle if a peloton, even more. The breakaway plan can make sense, but not by fooling around in a TT helmet in the middle of a peloton for a few hours first.