In too many games I've seen duelist players force armour+sheriff after losing pistol round, or not saving on the bonus round. This severely decreases the chance of winning the game due to the team's overall buy being 1 person weaker than it should be. Instead of 5 people having full armour+rifle, now 1 person has armour+marshall. Losing 3rd round will almost guarantee you lose the half if you already lost pistol, and losing 4th round after losing the bonus because the team is down 1 rifle when you won pistol has the same effect.

This problem is far more prevalent in lower ranks. As I've climbed, I notice that high elo duelists almost never ego buy when they shouldn't. If they do, then they at least can afford light armour+rifle for our full buy round, which is an almost negligible disadvantage. A lot of lower elo duelist mains I see have aim and general game sense that belong to a higher rank but are simply inting elo away by impatiently buying. I can guarantee to all the duelist players reading this that your rr will increase by a solid 50-150 just by purchasing normally, because you are not going to hero 1v5 on eco against full armour spectre.

TLDR: You are not TenZ, aceu, Faker, or whoever you think you are. Eco when the team needs to and win the full buy.