Each player has their own preference. I do not say that buying bonuses is bad, it is that I wanted to have a longer session with my fund, so with base spins, the amount that I need to spend for Bonus buy will take longer to exhaust with base spin. Even with base spin we also have a chance to win huge especially when we trigger bonus round.
The conclusion is the bonus feature is a short way to change 100 spins to 1 spin while the final result depends on luck, there is nothing wrong with this way the choice is in our hands

Yes, the bonus feature is the way to trim the time of spinning 100 spins but a repercussion of trimming the time is the removal of potential ROI or profit in every spin coming from the winning spin. In short;
Bonus buy = Trim the time of spinning and guarantee bonus round but it removes the possibility to recover a small profit per spin
Manual Spin = Can hit bonus round within a few spins and can recover some loss or gain profit per spin but it doesn't guarantee that bonus round will be hit after consuming the same amount of spin for bonus buy.
There's a cons and pros but the most important thing when deciding what to choose is what method you will enjoy the game. For me, Manual spin is the best to cherish every moment per spin.