I find that after more than a decade of listening to Martin Logan ESL’s, that 4’ of linear vertical dispersion is sufficient for frequencies >500Hz. Both by hearing and through ETF measurements, I don’t notice any significant ‘time smear’ from the 4’ linear wave launch from the panels.
From the front, I believe ribbons and ESL’s have pretty clean, and limited, vertical dispersion such that one is really listening to the segment of the line that the plane of the ear is on.
With really big panels (Monoliths) that go down to 250Hz or so, I find that even the mid-range is very controlled (although room modes begin to impinge more seriously).
What I’ve found totally lacking in ESL or other ribbon or planar designs is a matching mid-bass (60 – 350hz) dispersion and power curve. A >4’ line array of 7” or so drivers playing that range fixes that very nicely.
Therefore, I find that one of the biggest benefits of a line array is smooth and plentiful bass and mid-bass with dispersion characteristics that compliment Electrostats perfectly.