Question: Why does the fake case have wet paint if the swap was planned?
Answer: It's a while since I've seen the movie but if I'm remembering correctly, the meet was at short notice. Gregor had the case prepped for the swap but didn't have a chance to paint it in enough time for the paint to dry properly.
Chosen answer: It is true that it doesn't affect the plot. However Spence's (Bean's) failure can be contrasted with the professionalism of the main characters. Also it raises the possibility that whoever hired them isn't taking necessary precautions in planning and hiring. Earlier we heard Sam (De Niro) ask Vincent (Reno) if he was "labour or management" which suggests a theme of professional operators getting their hands dirty while their bosses play politics.