The concept of using S/O's to supplement heavy crews has been around, worldwide, for a loong time and has been very beneficial.
It allows the company to reduce costs, this should be self explanatory.
For the pilots, it's a great way to, as it were, do one's "apprenticeship". This is ideal if the time as S/O was around 2 or 3 years. This is a great opportunity for a pilot with less experience, to observe and LEARN.
I was fortunate enough to spend 2 years as an S/O on international long haul operations before getting my hands on a 737. It was an amazing experience, learnt a huge amount from all crewmembers involved. I'm sure it could get a bit stale after a few years, though.