When Shimano first introduced the current base design for indexed shifting (and I'm not including Positron or IGHs in this) back in about 1986 (aprox) they listed about 17 factors that were important to a clean chain run and reliable shifting (read as consumer acceptable). Among them was the guide pulley having sone side to side float to absorb the various tolerances of the parts that make up the system. The float had nothing to do with actual shifting, in fact the float ******* (slows down, in case that auto censor strikes "******") the shifting a little bit but the after shifting quiet chain run was more important to the market place (IMO). Remember that Shimano spent a lot of effort prior to Indexed shifting to quicken the chain's derailing from cog to cog, WRT lever travel. So a small bit of slower derailing wasn't a big deal.

Over the years Shimano has further refined the bits and increased shifting response, all the time with that guide pulley float still an aspect.

We see a lot of guide pulley bushings that are well worn out. Indicated by the pulley being able to rock or tilt off vertical. A good pulley will have some end play with nearly zero rocking. A worn pulley will rock and tilt. Andy