(Hero - Rogue) in Heroes 4

No one has ever died from having +50 to army movement.

+50%:) That's a significant difference.
The main point is to always have a thief in the army to speed it up, to maintain a fast pace of the game. What's the point of guarding a thief castle? The army will move slowly, and in the final battles, it won't be able to compete with the enemy army. Let's say your army moves 25 squares without a thief, while the enemy army moves 50% more. That is, in 2 days, you clear as much of the map as your opponent does in 1 day. In a week, you lose 3-4 days, then 6-8, and so on. Sometimes there is an opportunity to play without a speedster (although strong players advise always training one as quickly as possible), but on non-grass maps (deserts, snow), the lag in pace will be enormous.

Besides, why would you be stingy with 1500 coins for another thief? I'm convinced that these expenses will pay off very quickly.

2000-2500;) this is without taking into account the costs of leveling up through skill trees and warrior guilds.