As explained in some detail in this previous post Geneva Stop gear is designed to limit the maximum and minimum pressure from the mainspring to help control isochronism (varying time keeping as the watch runs down). But quite frequently the "nib" on the central portion is broken or the other section is missing altogether. This is, I suspect, normally down to previous repairers letting down the mainspring too fast which is easily done. It may also have just worn out, broken as a result of some other fault or been removed to give the watch a longer run time.
Yes it matters because the watch is not complete, so I always discount them a little. And yes if a contemporary mainspring was used it would most probably have a noticeable impact on time keeping.
But thing have changed in the way that mainsprings are manufactured; in the 1800's and into the twentieth century mainsprings were made equally thick through out their length so that as they unwound the pressure from the still wound coils delivered unequal pressure over the period.
A modern mainspring on the other hand is made so that the strength varies along its length to reduce variation in pressure. This significantly mitigates the problem so that on a 120 year old watch the affect on timekeeping is unlikely to be significant or noticeable without close monitoring.