Swivels on fly leaders?

Is the use of swivels in our terminal tackle a new development in our sport?

Fly fishers: do you tie a swivel into your fly-fishing leaders? 

This topic came up during an online exchange of messages, chap was setting up a leader, using a swivel to stop a metal bead sliding down his leader. 

Shot or weights on the leader is uncommon in the UK, but I have seen them used. 

I’ve honestly never seen anyone tie a swivel into a fly leader, nor has anyone offered me some for review. I suspect it’s against the rules in a lot of UK fishing?

I’ve seen swivels listed for sale – Rio and Scientific Anglers, I think – both list Micro Swivels. 

Am I missing something? So, fishing sub-surface, should I try replacing tippet-rings with tiny swivels?