The theory of not paying commissions is that salespeople will be incentivized the same way other employees are. You don't code on commission and tend not to coast once a revenue number is hit.
I've met quite a few software developers who pad their estimates heavily and then coast when the job is complete.
Which is great for everyone. The company gets predictable estimates, the developer gets a stress-free life. A shorter but unmet deadline is worse for everyone, specially the company, even if the product is still delivered sooner than the padded deadline.