And does that rattle actually cause a problem?
Yes. If anything is not fixed enough not to rattle, it will become more and more loose over time and will eventually fail. Considering Retina Display replacement costs ~750 USD I ****ing don't like it. Even if Apple replace it for free (how they do for the recent models
ONLY after huge number of complaints and media coverage, NOT because they're thoughtful about their clients), this is like the minimum they must do. This has nothing to do with me being pleased, it might only have something to do with me not being too angry at Apple. When I have to change my plans in order to visit a service centre, when I have to live without my main laptop for several days, when I have to worry if they leave data untouched or not - this does cause a problem, yes.
Not to mention others asking: have you heard this click? is your laptop broken? For machine that costs that much it's unacceptable.
Also considering my Vaio 2009, MBA late 2011, MBP late 2011, rMBP late 2013, Lenovo 2014 do not rattle... Apple lowers the bar the deepest they can.
P. S. Funny thing - this rMBP 15" mid-2015 also has semi-broken key - it stucks semi-pressed diagonally and clicks when you touch it. It still works however, just differs from other keys and clicks loudly. No problem, right?
Laptop is 2 months old. Of course I will contact service centre and fix it (most likely for free). But damn... I already know the guys working there and they know me. This seems to be an unhealthy relationship between a premium-laptop-buyer and a premium-laptop-service-centre.