I add these because it seems like the right thing to do. I have added several to roundabout flares, and also to nodes where a road changes to dual carriageway.

Topologically, this is a node connected to three ways:
* a oneway=yes entering the node
* a oneway=yes leaving the node
* a oneway=no

The roundabout flare is topologically equivalent to t-junction where a two-way road meets a single one-way road.

Topologically, the roundabout flares look like a one-way road which leaves the roundabout, then rejoins it. Topologically it looks like a shortcut. The turn restriction is the only thing to distinguish this from a single one-way shortcut.

In practice various routers would treat the flares as a shortcut if the flares did not have a turn restriction:

OSRM does (last I checked) happily route off a roundabout, u-turn from exit flare to entry flare, and back on to the roundabout. OSM will take this route in preference to waiting at any traffic lights on the roundabout.

OSMAND would give a "keep right" instruction (ie turn onto the entry flare) if you take the wrong exit off a roundabout. It sees the u-turn as a shortcut back onto the roundabout for you to take the correct exit.

It is even more stark in the single-to-dual-carriageway transition. If some other turn restriction leaves you going in the wrong direction along a dual carriageway, OSRM will happily plan a u-turn from one carriageway onto the other at the joining node.

Without a turn restriction, how would any router know any different?