Well it depends on the probability you'll actually fly the booking doesn't it!

I've used £20 Ryanair bookings as backup to "posh airline" flights, a few times, when I absolutely have to get to the destination that day (for example positioning flights before an expensive long-haul itinerary). I reckon in the long run I'll probably use about 10% of the bookings I make like this (it's not happened yet). I therefore don't purchase baggage or any of the other stuff that makes it more civilised - on the principle that financially I'll come out ahead over time this way, and if I'm ever in the position of *needing* the Ryanair "backup" flight I've booked, I'll be jolly grateful for the opportunity to pay £200 for my bags/seat/etc and actually reach my destination.