Just not robust enough
I do really like this gilet - can be worn for almost anything (including keeping warm in the office). It is very effective at providing a convenient middle-ground where a long-sleeved top (jumper, fleece, jacket) would be too warm, but a little something additional is required. It would be especially useful as that first item to go on when climbing and a t-shirt is no longer quite enough - especially when said t-shirt is a bit damp and starting to cool down - but you are still generating quite a lot of heat and sweat
...However, I wore it for 4 days with a rucksack and noted that the back was covered in hundreds of little pills / bobbles of material as the surface degraded. So, great for general use (that chest pocket stores a kindle quite nicely), extremely breathable (only notice quite how good this is when you try a proper windblock gilet instead), not waterproof and in fact barely water repellent...but if it is not tough enough to wear a rucksack on top of then it just isn't good enough.