I didn't send in all of my payslips, but I made quite a number of copies of the different payslips I had from every company. If you have business cards from any of the jobs you did, attach them too. The more "evidence" you have to show, the better.
No, not necessary in most cases. If you work a regular full-time, salaried position, have the required recent letter of reference and tax documents, then you do not need the pay stubs.
For people who work varied hours every week, and who are paid on a hourly (rather than salary) basis, pay stubs are useful.
If you don't have all the required documents, then paystubs MAY help (any time you're missing a required document, tho, increases your risk of refusal). But they should not be considered "substitute" documents, only documents of last resort.