Cremator,
Do not under estimate the usefulness of controlling RPM. It is not meant for fighting - it is meant for cruising. The E6B recommended settings are according to the pilot notes. In the game there is no problem running full throttle and reduced RPM.
Reducing RPM (while throttle at max) lowers you fuel consumption. It also reduces your speed, but not proportionally. The result is a significant increase in range if you are willing to travel at less than max power. In the big american radials this is a very significant boost in range. The La7 and Yaks have a decent range if you are willing to travel at less than their max speed (they are very fast, so you will only cruise like a normal plane...). So, you can use reduced RPM for cruising on the way to the destination, while loitering and when you RTB on fumes.
The other use for minimal RPM is gliding. Planes glide MUCH better when the prop is windmilling less. I have glided planes a full sector to a safe landing several times - either following damage or loss of fuel.