Apple CEO Tim Cook claims allowing App Store developers link to their own third-party marketplaces and payment systems would be a "terrible" idea. 

As reported by Bloomberg, Cook was called to the stand last week during the ongoing Epic Games v. Apple trial and was asked to outline why Apple is reluctant to give its developers and users more freedom when it comes to spending.

It's a critical issue given the legal skirmish between Epic and Apple sparked into life over the latter's decision to remove Fortnite from the App Store after Epic breached platform guidelines by implementing a third-party payment method of its own. 

When asked what would be the problem with letting App Store developers give their customers another way to buy virtual goods outside of Apple's platform, Cook said "it would be a huge convenience issue, but also the fraud issues would go up" because customers would be required to enter payment information multiple times.

He also indicated that letting developers sell virtual goods -- such as V-Bucks, the in-game currency in Fortnite -- outside of the App Store would see Apple "give up our total return on our IP." 

During his time on the stand, Cook fielded questions about App Store curation, profits, and other policies, and indicated it was his gut feeling that the App Store was profitable, but that Apple was unable to back up his assertions with actual data. 

"I have a feel, if you will," said Cook, suggesting the platform was indeed making money, before adding that he "thinks" in-app purchases bring in the vast majority of App Store revenue. 

Both Epic and Apple are due to lay out their final arguments later today. The judge overseeing the case should deliver a ruling in the coming months.