That's interesting. In all the literature I have read, it seems to say that a monopole over a ground is exactly like a dipole...
typical quote from one such writeup --
"The quarter-wave monopole antenna is a single-element antenna fed at one end, that behaves as a dipole antenna. It is formed by a conductor lambda/4 in length, fed in the lower end, which is near a conductive surface which works as a reflector (see effect of ground) and is an example of a Marconi antenna. The current in the reflected image has the same direction and phase as the current in the real antenna. The quarter-wave conductor and its image together form a half-wave dipole that radiates only in the upper half of space..."
No doubt this comparison is made with a dipole to facilitate easier understanding. But then, what is the difference of monopole vs. dipole with regard to current & voltage distributions ?
Also, since the monopole is much more unbalanced by nature of its design, surely this will result in higher likelihood of unbalanced currents flowing in the co-axials shield and hence result in unwanted radiation from the shield as well ?
I would think this would compel one to definitely use a current balun for the monopole, even more so than for the dipole ! Your point about a "virtual ground" isn't really clear.