Given that rugby is often played on muddy, soggy pitches (especially November-December time), plastic moulded studs on football boots are pretty useless as they are much smaller and shorter and give next to nothing grip, so boys are just likely to keep sliding in the mud. Also, rugby boots' studs are completely different shape to football ones, they are much wider and rounded at the end, while football studs (both metal and plastic) tend to have sharp edges and are more likely to result in a cut to an opponent's leg. DS had a number of unintentional encounters with his mates' or opponents' rugby studs and I'm glad they were not the sharp football ones.
We started off with a set of studs to swap between football and rugby (i.e. unscrew one set and put the other set in as required) but after the first few weeks of having to remove the caked-on mud first three times a week, I decided to spend ca. £20 on rugby boots from a well-known cheap sports retailer.