To the original OP's question.
No it is not a very wise idea.
You can fly quite happily around europe for days with out ever being asked to produce it. And even get back into the UK without border control turning up.
But if you do require to produce it and you don't have it all hell will occur and in some countries France for example they will put you in jail until everything is sorted out. Also some countries Holland for example if you don't have it on you walking around the streets you will get an on the spot fine of something like 100 euros, driving license doesn't cut the mustard either.
Saying that though even if you don't have a passport some countries will allow you to enter as long as they know about it prior to you arriving. The Dutch seem quite cool about this. An elderly pax on a charter (2 day wedding) didn't have a passport we phoned them and faxed through some docs and when arriving they issue her a letter. Getting back in the UK though was more of an issue but one of the pax had contacts, which I wasn't happy about because if they do take exception to you bring someone in without a passport the PIC is liable for 2k fine.