I am well aware that strong open-source contributions are an advantage. What I find objectionable is not this advantage. It's the fact that failing to secure this advantage is being portrayed as an obvious professional failure that can be simply rectified by working an extra few hours a night on side projects.
It is propaganda from open source people, many of who have achieved considerably less in their open source projects than a casual inspection might suggest. Usually I come away from people's open source projects with a cheerful acknowledgement that this person (a) likes cool stuff and (b) appears to be able to write working code of a kind. This is good - but not that good.
There's a big difference between 'I wrote some code, look at me' and 'I contributed a competitive register allocator to LLVM' or 'I rewrote the scheduling algorithm for major open source FooOS'. The latter are 'strong contributions' and deserve the 'massive advantage' of which you speak. A handful of pet projects constitute a code sample that suggests that the person can form syntactically correct code.
The underlying issue is that from a hiring perspective, all other things being equal, a candidate that can provably show things is ahead of the guy who simple says he can. Even if "we" all sit around and minimize that as some trivial nonsense that anyone could do and that it proves, at best, they can form "syntactically correct code", the fundamentals of the situation haven't changed. Showing is better than saying. It always will be.
There is an undercurrent of a strawman, as well, in this conversation. I don't think anyone out there is taking actual trivial github stuff and basing hiring decisions on it. But I wouldn't be surprised if some superficial github stuff haven't gotten people past a resume screen that they wouldn't have otherwise gotten past.
When someone says he did something, a github commit log showing it is definitely better than just him saying he did it. However, the point you actively dismissed was that a commit log of some mundane contributions is much less impressive than a resume describing work requiring a lot of skill.
'I wrote some code, look at me'
Throwing code over the fence is not what open-source is about. I look at it as -- is this person capable of building a product for other people? Is he a team player?
That's not a skill that comes so easily and requires lots of experience or natural talent at other things than churning out code.