Have a look in CAP 168. You will see that the lamps are required to produce a certain intensity to a particular pattern. An Omni-directional lamp won't conform to this pattern. The requirement is so that on final, a slight misalignment of the approach will produce a slightly mis-matched intensity. When on the centreline, the intensities of each side will be equal.
We had a similar issue in the 1980s with taxiway lighting at CATIII aerodromes. Initially, these were Omni-directional fittings with switchable routes. For those of us who grubbed about on the ground, it was easy to see what routes had been selected and therefore we could avoid aircraft that had simply been instructed to 'follow the greens'. ICAO required that taxiway lights should produce a pattern viewed fore and aft, with a different spec for lamps on a curve. This made it impossible to see from a vehicle which route had been selected. I suggested drilling a hole in the side of the fitting but I think they thought I was taking the proverbial.
When I teach the night rating, I major on this difficulty in seeing the runway from an oblique angle. What I teach is to look out for other features, principally Apron lighting which usually stands out very well, even at a smaller aerodrome like Newquay. I say 'the runway is that dark linear hole in amongst all the lights'. It appears as if by magic as you turn final. BTW, the approach and PAPI lamps have similar narrow output patterns. Any Omni-directional runway lighting is probably non-ICAO compliant.
Makes for good bed-time reading, does CAP 168. Gets you off in no time!
TOO