Side skirts??... as far as I know a standard Murena hasn't any sideskirts
A standard Murena does have 'side-skirts' in so much as it has fibre glass sill covers to hide or clothe the galvanised chassis, but they were black and recessed, unlike the 'S' versions which protrude. Whilst the standard covers simply rivet on, you need brackets to attach the 'S' sills, and these rust through, so many need or will have had to repair or replace them by now.
Whilst the 'S' sills can look better in some cases, one reason the designer chose the original black recessed type was that they reduce the apparent height of the car when viewed from the side. The eye only sees the coloured section between the roof and bottom of door/wings, and as the car is quite short for it's height, it gives a better proportional and pleasing vision. The 'S' sills can accentuate the cars 'dumpy' looks.
A second problem with the 'S' sills is a practical one - Most of the cars I have seen with them, the paint work on them suffers badly from scuffing as peoples shoes catch or rub them on entering and exiting, so after a while they look bad. You simply don't get this with the standard sill.
Roy