While bonus area is very bad compared to Stage 1-3, I think you're doing an unfair comparisson.
The problem here isn't the poor effectiveness of B.A., but the game-breaking unbalanced effectiveness of 1-3.

We all admit that Stage 1-3 is frankly too unbalanced, that it pretty much lames the spirit of the game (which is most definitely not allowing a player to reach top100 power levels in a month by staying at less than 10% of game completion).

If we were to do our maths on most stages, we'd probably get numbers pretty close to those in the bonus area.

Originally Posted by Zerana

Yeah, the bonus area is really bad. Truthfully GHQ is a lot better. It requires 20 stamina, but the odds are a lot higher that you will get a triple hit. Truthfully I wish we could go back to a different area like in the other games and hopefully some day that might happen.

I will agree with you on that. However, I find that, right now, outside of the Chance Time mission (to get R-Gachas for Kurito), I'm happier farming Pero and Exp from the Bonus Area (which is the advantage that zone has over GHQ).

Nowadays I complete all three of the Pie, Stamina, and Chance missions, and invest all the pero into N-Gacha that net me at least 2 of the 3 Flirt missions. Outside of that, I manage at least all the other one-star missions.

Does it hurt my game? Maybe. Not getting as many R-WC as before. However, once you have leveled up all N-Cards, 90% of your Chance Times will only result in extra pero.


Resuming:
Just comparing Bonus Area against Stage 1-3 is completely unfair and unrealistic, seeing as Stage 1-3 is acknowledged as a broken stage in terms of balance. I'm pretty sure the whole change from "random card levels up" to "get some bonus pero" for Chance Times was implemented just to mitigate that stage some.
BA is a lot more balanced in comparisson with any other stage in the game but 1-3.

GHQ has better chances than BA, but it costs twice the stamina, and delivers a lot less pero/exp rewards (BA costs 10sta and rewards equal exp and 4x pero, while GHQ costs 20sta and rewards 0.75exp and 3x pero).
There is a nice balance there: Choose between more spin rewards and faster grinding/farming.

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I don't think having access to all stages is a good thing.
It actually has the potential o laming the game, exactly for the same reason you're asking for that access: 1sta spins mean that even the atrocious rate of 4% from BA would yield almost 60 of each triple-spin. That's three times a guaranteed mission roster (plus 60 lvls/18k~60k pero, just from chancetimes).

However, a kind of sub-bonus area for each stage would be interesting. They'd only need to raise the current BA's rates a bit (Say, around same rates as best substage overall) and then each subBA would get proportionate values acording to the stamina cost (if BA costs 10sta, and has a 9% rate, then BA1 could cost 4sta and have 4% rate; while BA2 could cost 8sta and have a 7% rate).