I get what you are saying as far as unit count for G4 or G5 timing.
However the bomber can help break Russian lines on G3 sometimes before the reinforcements from the East arrive.
Also can cause Allies to build more escorts or fewer transports and delay landings a round which means fewer units used for trades so more can move east.
There is a gap in follow up reinforcements that occurs as a result of this as you point out, but this is the trade off for higher speed.
The bomber threatens sea zones 3 and 13 that could otherwise be safe moves for Allies without it. It also helps threaten Egypt or Trans Jordan attack without needing to have fighters in range of those territories which puts them out of position in other areas if they are.