When I have to.
I replaced my 2004 FS with a NOS 2008 frame in 2015 because I thought I had ovalised the headtube (I hadn't, I had pulled the fork dropout off the carbon lower). Lack of funds meant I had a tight budget but thanks to the hardtail I had a long time to pick up bargains (~1000 euro for an excellent spec XC bike).
My hardtail needs some new cables and the fork could do with a service.
Original thread on the hardtail
I replaced the commuterised pompino when my hip was playing up and on the indirect advice of the consultant, and seeing how the Pomp had paid for itself many times over I did "splash" out on a top of the range PX-LR so I can keep cycling.
I don't feel I'm good enough or on the bike enough to justify spending a fortune on my hobby.
That and I'm a tightwad.