The class balance (pick rate) was pretty good during the open beta of BF6. I think the synergy bonus is incentive enough. It should just be something small, not something that gives a clear advantage.
Even 10 extra attachment points would convince me to change to a different class just to make my preferred weapon stronger.. making the class system more redundant. As a player i don't really want that. I want a meaningful class system that let me choose class based on the needs of my squad, without forcing me to pick a specific weapon.
With you on that. I would support that during open weapon playlist
