That mentality is completely off. It's just
plain wrong, completely opposite, not having set bonuses is just being able to upgrade that slot trough the entire raid tier plus having option to upgrade it from various sources.
To make better example, with tier sets you sort gear by:
1. Potential upgrades
2. Trash that takes up tier slot
Without it, you only have potential upgrades.
Without tier sets you have 15/16 gear slots.
With tier sets you have 11/12 gear slots and 4 big sockets.
Exactly like in this comic:
https://www.darklegacycomics.com/612