The role of a tailgunner as an observer/lookout can be easily overemphasised. He wasn't as useful as you might suppose...
If you have ever flown at night at altitude in a commercial or military aircraft - then you know that at altitude, for a considerable part of the night after "dusk" the WESTERN horizon is
still illuminated to a great degree...I.E. to the REAR of an aircraft heading
to the Continent...
After the wee small hours, the EASTERN horizon is slowly illuminated by the upcoming dawn...I.E. to the REAR of an aircraft returning from the Continent to England...
So fighters attacking a bomber from the rear at its altitude WILL to a certain degree and depending on the light be silhoutted against the glow as it heads TO a target on the Continent....but only well towards dawn on the return leg will the tail gunner be able to eyeball the target...
That's two very narrow time windows on really quite long flights
At OTHER times, he's relying on seeing a dark aircraft against a dark background!
He COULD possibly pick out an attacker by the flare of his exhaust stubs - some of these were VERY obvious at night!...but the Germans learned this too, and equiped their night fighters with exhaust flame baffles
It was these ruining the overall "tune of the exhaust as well as the growing weight of the aircraft, that reduced the overall top speed of the Me110G to significantly slower than a Me110C.
BUT....
...an aircraft passing BELOW a bomber, with darkened upper surfaces, will be virtually invisible; the bomber crew are looking down into the dark with no form of light to silhouette/illuminate the fighter!
"
Schrage Musik" as a tactic was a step forward....for it obviated the need to build up ENOUGH speed to actually outmanouver your opponent
You have to remember that MOST "nightfighter attacks" were actually
long stern chases once a jaeger was tasked to a target by his ground controller. The LW pilot THEN had to be vectored onto the target until he was in range for his own radar to take over...
THEN as well he was having to climb to around the same altitude and build up enough speed to catch the target, having been loitering on cruise in his respective Kamhuber Line box.
It was VERY difficult for most nightfighters to build up enough speed to actually outmanouver a target for an attack from the beam or any "known" weak point. Especially earlyish nightfighters whos power output was compromised by their flame baffles. Those little short exhaust stubs that look like an afterthought on inline engines had a VERY precise length and function!
So - the "
schrage musik" installation was the LW's practical way of delivering a devastating attack that DIDN'T mean outmanouvering the target - they JUST had to overhaul it and slowly pass beneath it!
Part of the overall losses to
schrage musik HAS to be down to the way the numbers averaged out - for ANOTHER problem with the Kamhuber Line was that...that pilot THEN had to return to HIS box to be tasked again to a new target by HIS controller as one passed through it.
Therefore a nightfighter could only attempt to chase three or four bogies in a night...
but if he saved fuel AND clock-time by a simple passing attack on a bomber from rear to nose - he could perhaps squeeze one MORE engagement out of his fuel load and the Bomber Stream before his evening's activity ended This is also why nightfighters gave up on bombers that tried to evade - they couldn't aford to waste the fuel and distance it meant chasing them even further down-range!
This was one of the main issues with the German air defence - the Kamhuber Line could ONLY "do" a certain number of attacks on intruding aircraft in a night. Therefore there was a relatively fixed percentage of Bomber Command aircraft in a raid could be targeted in a night. Hence the initial tactic of the Bomber Stream - to scatter an RAF raid in time across the K-Line And thus make it harder for LW nightfighters to move back and forth across the landscape in and out of position AND manage to be in the right place at the right time to be tasked to a new bogie.
And because of that only-barely-marginal speed advantage over their targets...if a lancaster pilot dived and corkscrewed his aircraft to build up speed, he was uncatchable - and most jaegers knew not to waste time trying. For THEN it was going to turn into exactly the kind of evenly-paced night time dogfight they didn't have any power advantage in
This is incidently ALSO one of the reasons for the deletion of ventral turrets (and occasional dorsal!) in several RAF bomber designs; that they were covering angles of attack that nightfighters couldn't build up enough speed to outfly the bomber and attack from
It was also in turn the idea behind the deletion of ALL but tail armament on designs like the Vickers Windsor and the high-altitude prototypes of the late-model Wellingtons...and THOSE fired by remote control from the pressurized cockpit
A tail
gunner didn't need to be IN the tail to SPOT an attacker...given that this would be so difficult...he could fight his guns from elsewhere in the aircraft...
...taking this a step further - was the Victory Bomber deisgn of Barnes Wallis in the early years of the war, before he became involved in the above two projects; the six-engined Victory (Vickers Victoria?
) would spiral up to its service ceiling over the UK, THEN head for the continent...mandating any even HIGH-level interceptor to pursue them UP to their ceiling and up to their
already built-up speed advantage
...
a chase that could take a fighter HUNDREDS of miles downrange In THOSE circumstances..... the Victory would again
only need a tailgun...
or no guns at all 
