Good question, Andre!
My main goal, as Product Manager for APIs, is to have an easy way for developers to get Wikimedia content and data with our public APIs.
So, a single portal that they can use to sign up for an account, get an OAuth key, read tutorial and example code, and then start coding, using API reference documentation as they're writing code.
This goal isn't well-served as part of a standalone site, shunted off to the side. That would make it hard for people to find the functionality I'm trying to share. It works best if it's part of a more integrated portal that is the official place we want all developers for Wikimedia to start.
It feels like we have had this idea of a developer portal multiple times, and that it's generally agreed that it would be good for the Foundation and the movement. I think we can work together to make that happen.
I realise that's not a detailed description of what content would go onto a developer portal. If I were to start somewhere, I'd start with How to contribute; content that was more than a click or two away from that page might not be what we want new developers to start off with.
I'd say in general doc.wikimedia.org stuff wouldn't make sense to include; that documenting MediaWiki on mediawiki.org makes a lot of sense; and that a lot of the content on mediawiki.org that's not actually about installing, extending, or hacking MediaWiki might make sense to transfer. And I'd put an asterisk on the API documentation for MediaWiki and MW extensions, since they'd be important both for self-hosted MW and for people trying to work with Wikimedia sites.