For Valve, this is actually a more open way than they have historically done. Typically Valve publicly gives very little to no information on a project until it's done or almost done.

The question would be when the amount of people who complete newcomers will have fun playing around with will be too small, thus pushing more skilled players to be in the same game.

In TF2 even quickplay games were/are amazing because the game itself still is fun with an absolutely uncoordinated team, and random crits alongside players seeking action will often keep more players from always pubstomping, and it is so chaotic because the team size is big enough for uncoordinated pushes to get you somewhere.

I don't see how a MOBA, which has a very small team size by design and generally is more strategic, will find a similar footing in terms of new player friendliness.

Yeap. Anybody can invite any friend to the beta and that's it.

Althought I agree, at that point just make it a truly open beta.