I’ve driven nothing but manual cars of various different stripes for the past 20 years and changed gear going on either the sound of the engine or at the point where it ran out of puff.
I can’t think of any time I’ve actually needed to do that, whether I’ve been in a hurry or pulling.
Back in the day 
I’d been a passenger in my parents car for years before I learned to drive. In that car, I already knew what the engine sounded like at the point my parents changed gear.
The operative word is ‘safely’.
What seems to be forgotten is that, generally speaking, by the time an engine is hitting that red line it is well past its peak power output; for example:
http://www.motorcycle.com/gallery/gallery.php/d/27816-7/Dyno_Chart_07_Suzuki_DR650.jpg
Accepting you need to have an idea of the power curve of your particular engine, you get maximum performance keeping the engine speed in the max power zone as far as possible considering the number of gears available. You might, for example, need to take it to the red line so when you change up so engine RPM doesn’t drop too far down the other side…
I drive a manual and use the tachometer occasionally. It comes in handy in unusual situations where it is hard to judge engine speed and hard to judge vehicle speed. A good example is when approaching the city and going through underpasses and tunnels in heavy traffic. The lighting is weird, the road is unfamiliar, and I’m interacting primarily with the cloud of other drivers around me rather than with the roadway shooting by. It’s noisy and there is a lot of echo. There are trucks and motorcycles and “hot rod” cars whose engine sounds hide mine. I’m trying to speed up or slow down for a lane change or to get ready to exit. At these times I really don’t have a good feel for how appropriate my engine speed is for what I am about to do next, but it may be important to have high enough rpm to have power available, or I may feel embarrassed by sounding like a vacuum cleaner. I’ll want to have for example 2000 rpm for steady cruising or 3000 if I’m about to try to accelerate to get past a car blocking the exit lane. Or if things feel weird I may need the gage to help me realize it’s because I’m at 700 rpm.
The tachometer is much appreciated then. But the rest of the time it’s not important and I ignore it.
In an automatic transmission car, there’s no point, I’m sure. I think it’s supposed to look cool, like having an “X” and several zeros in the car’s model name. And, yeah, maybe, it’s more costly to make two versions of the instrument panel – but I’m sure I’ve seen my friend’s car with a tachometer and an automatic transmission, the instrument panel being a big LCD display.