canadianna
Sign it.
It has been part of the recruiting requirement for a few years now. I doubt you will be allowed to join without signing it.
Just a quick recap of the history behind this issue. In the mid 90s CX created a back door way of getting CNs to work on much lower salary - ie Aircrew Services Limited (ASL), flying only freighters. Until ASL was created all freighter flying was mixed with pax flying in CX. In late 99 CX/AOA gave a one off opportunity to allow the ASL guys a chance to join the CX seniority list, wef 1/1/00 but remain in rank. Some ASL guys chose not to - either because they were close to CX RA of 55(vs ASL RA of 60) or because they wanted to remain in ASL and get a quick command. This was perceived as a CRM issue. As CX management perceive it is only an issue with anyone that joined prior to the 1/1/00 date, they have required new joiners to sign a form saying they will fly the freighter.
So to cut a long story short, you will not be perceived in a bad light for signing this waiver. If you want the job, sign it and join!