I guess you can tell from my screen-name where my loyalties lie on this. I loved my MD/PhD, but I didn't do it for the application purposes (didn't even know about Rad Onc when I started med school). I knew that I loved science, but I wasn't so sure about the clinical side, so the MD/PhD was an obvious fit. I have a brother who is an incredibly successful MD-only bench researcher, and he recommended the combined program for me because he said it was the one time in life when I could study the skills to be a good researcher, without having other commitments. He has always had to balance learning research skills with clinical work, and feels that the gaps in his knowledge have been harder to fill as a result.
I spent my PhD years focusing on gaining statistical skills, study design, writing as many papers and giving as many presentations as I could, and writing a grant. Although all of those activities look good on a CV, the best thing about my training is that I really feel ready to be an academic researcher. I don't need someone to do the analyses for me, or write the papers for me, or write the grant application. I've got the experience, and could step into a K grant now. I may need to spend five years in residency training, but all of those skills will serve me well even after that.
Oh, and about the $$... for most people, the opportunity-cost of missing attending money for the few extra years balances out with not having loans to pay for med school. I don't know if Rad Onc is an exception, since we have higher attending salaries, but I do know that my life is a lot more sane right now compared to my cohort who are wholly overloaded with debt. They all talk about moonlighting to make up as much cash as possible.... I know that Rad Onc has fairly good hours, but I'd like to spend my off time studying, doing research, and enjoying myself. Not working a second job.