-w warns about things looking like an error, in short things you can but should not do. Fine for hacks, but anything more than 10 lines is already way past that point IMHO

Do you really believe that when your test code matures to a bigger and bigger program that on a fine sunny day you will sit down and convert that code to use strict and -w? Probably not. That's why it is better to do "the right thing" from the beginning and get used to it