Why should PPC spend approach break-even ACOS?

I’ve been advised to increase my PPC spend until the campaigns reach the break-even ACOS of their ASINs but I don’t totally understand why.

I understand that maximizing sales improves organic position and that even a sale with a 1% margin is profitable. Also, taking as much market share as possible helps the product snowball – customers like to see things like “1K+ bought last month”, Amazon’s Choice badge, etc. What I don’t understand is this:

• Let’s say an established ASIN has a break-even ACOS of 40%.
• A campaign for this ASIN sells 100 units/30 days with an average ACOS of 20%.
• The bids are increased. Now the campaign is selling 200 units/30 days with an average ACOS of 39%

How is it better to have a barely profitable campaign over a solidly profitable one?

To me, it makes more sense to find the sweet spot where the campaign is delivering the most sales with the greatest profit, but the consultant I’ve worked with insists that, in most cases, it’s better to aim for just below the break-even point.