The only thing that *is* mandatory is proof of health insurance - and in practice, the EHIC card is more than enough.
I have that and have used it. However it was a hassle and I just presumed it would be easier all round to get a PESEL number as well as that because it obviously suits their paperwork better.
Sure, you need one for a mobile phone contract - but this isn't really mandatory in day to day life.
That's true, I have just always found the contract phones work out cheaper than pay-as-you-go and I find the bills easier to manage when I know roughly what I am going to be paying a month. But I guess I will have to do my 5 years first!
It's really just the same as in the UK - you don't *need* an NI number.
I am aware of that, but, like in the UK, I can imagine it makes many aspects of your life much easier.
So now I need to go off and get this residency card.