I'll put in multiples so they keep the range-to-place-BMs up even when the formation has multiple BMs on it. It's also nice to have a little bit of a boost if your Warbikes end up in FF or your Skorchas end up in CC. (Statistically, it's such a small difference that it's irrational but it makes me feel better anyway.

) The total number of buggies depends on the size of the KoS but never many - something along the lines of 2 in a normal, 3/Big, 4/Uge or less.
I defended Buggies for a long time. I used to take an all-purpose KoS that was mostly Buggies with a few Warbikes and Skorchas. It was always somewhat useful but suffered from being jack of all trades, master of none. As a result, I can't recall the last time I took a majority buggy KoS.
The problem to my mind is that they are just a shade worse than the bikes and skorchas. I don't know what you could modify to make them equal but not better, and I don't know how you could build an ork formation that would allow you to give a price difference based on unit type.