Maybe I'm a little naive but I wouldn't call that a write off. The seat tube is going to take downward force mostly so I wouldnt expect it to split randomly.
I have seen in the mechanics section, whenever people find cracks, ripples on the downtube by the headseat etc, the advice given is usually 'get rid of it' rather than 'this is how I would fix it'.
In my life I've had a seatpost snap on me which I knew was coming as it was slowly bending towards the back wheel and when it did go, I could see the fault in the casting process.
Have people had catastrophic splits in frames from damaged frames whilst riding, or is it just pre-empting the end and saving themselves any inevitable injury by paying the few hundred quid for a replacement frame?