For prints up to 11x14 in the Leitz Focomat V35 I cannot see a difference in sharpness between glassless and glass unless I start looking at the print through a strong magnifying glass. Now, who looks at prints this way? Honestly, the prints are so sharp that I really don't care either, but that is unique to this enlarger for me. The slight loss of sharpness is nothing compared to needing to spot the hell out of each print (something I absolutely loathe to do).
For prints sized 8x10 and up in my Omega I see a clear difference in sharpness, and here the extra effort is definitely worth it. I use a 4x5 glass carrier and mask it off depending on negative size.
I don't think all enlarger neg carriers are made equal.