I completely agree. It can help but it's not a good idea to rely on it exclusively.

The biggest problem for me personally is that it's learning without meaning or purpose.

There's no situation where I'd use a sentence like "This letter, the dog wrote it."

Even when the sentences make sense they are removed from context and purpose - it's like practising swimming on dry land.

A shameless plug - I created a browser extension that helps with that somewhat. It's called Duolingo Ninja (duolingoninja.com) and it basically translates web pages you visit based on the Duolingo vocabulary you already know. So, reading becomes actually meaningful.