Synthetics are great for pack and forget negligent garment use – stick it in your pack and don’t fret if it gets damp. Put on over damp tops, no problem. Easy to wash, unlike down. For most UK use, Primaloft and similar synthetics make lots more sense than down, even shelled down for those reasons, but down out-sells synthetics by a way because mostly people just wear them to the pub. Over winter, I always carry a lightweight synthetic jacket in my pack along with a superlightweight survival bag. It’s saved my shivering butt on numerous occasions – usually someone else’s endless mechanical in the middle of nowhere. No way would I treat a down jacket the same way unless it was consistently dry and probably sub-zero too.
So, personally, I’d say get one, but I’d go for a jacket rather than a gilet. The idea of a ‘warm gilet’ is a bit of a pipedream ime, simply because you lose a lot of heat from your forearms in particular.