Any material after sliding wears out. It is true and with your shoes. Lens is no different. Leica lenses are not exception. In average, after use (focusing) Summicron R50 mm, around 10000 times you can notice wear. Many just do not pay attention (and do not need) so they think no wear.
If you think about the best lenses Nikon produced, Micro 2.8/55 will be around. This Nikkor, Leica 60mm, or Canon, no visible difference on photographs. Even Nikkor is with floating element that rearanges glasses in order to hold performances and at far and at near. I have that lens around 6 years and I am very pleased with. It found its permanent place on one of my F6 Nikon. I just did something bad to that lens so even it works very well I am thinking to get a new one. Even new Zeiss Macro 50 mm F2 is in play I just cannot find what Zeiss could better over Nikkor. Even engraving on Nikkor is so nice that nor Leica nor Zeiss can come even and close.
Nikkor 2.8/55 Micro is a must if you intend to make techically high end photographs from 35 mm. The only problem some reports is that apperture blades gets oil. I think this moment it is due to careless storage, will examine, and if it is up to user I will not think twice to get a new Nikkor before a new Zeiss.