Whether cosmetic or functional is more from a shooter or usage standpoint. Some clients say there has been a reflection in some guns without serrations and other say no issue. Mostly this would be on what the top of the slide is finished with. From shooting IPSC and IDPA, and various other clients who shoot a lot, and if the top is dull or flat finish the chances would be little or no benefit, but is there is any possible shine or reflection potential in the top of the slide the potential for benefit is greater.
I do know from clients and this may be an extreme a highly polished blue or hard chrome, or polished stainless steel I have heard comments on shooters losing the front sight. But these are casual or 100 round a month types, not the 100 round a day shooters.
Again, just some thought and comments from shooters, clients and folks I have seen on the range for years.