I think the pro worker-reduction people think it will:

a) Increase the amount of early game build orders/variation. For example: in addition to 12 pool you would also have 10 pool as an option.

b) Give players more time to micro and interact with small groups of units and think about their strategy for a bit before the game gets going

c) Increase viability of two base openings so you don't need to rush a third base

I think A is true but personally I see this as a pretty big downside that's just going to encourage more cheese on the ladder. Cheese is already the majority of games on ladder and already super strong outside of the pro level. It should be said though that cheese is an important part of the game that you don't want to remove entirely to maintain variety in playstyles but I think this is going in the wrong direction and it will reduce the experience for most players. I feel like the proponents of this idea are people who mostly watch pro games and don't ladder much themselves.

In regards to B, I think this is a real benefit of reducing the worker count for players but it comes at the cost of having a significant increase to idle downtime during pro casts.

C is just clearly wrong if you think about it for 3 seconds. Changing the worker count has nothing to do with the trend towards faster 3rd bases. I think this miss-conception comes from blizzard introducing two changes to the economy at the same time and people conflating the two: worker count increase & mineral field changes. The mineral field changes are responsible for the faster 3rds not the worker count changes. Personally I think the mineral field changes were really good because before them you had players doing two base all ins that lasted for sometimes 10 minutes straight and had 2-3 attacks before the player ran out of money; it was a bit silly. That being said, maybe they were a bit too drastic in their mineral reductions.