A friend working as a guide in Chamonix has belayed clients with micro traxion in place of reverso/hms for many years.
Having heard many stories of various guiding practices around the world (and knowing some guides in Chamonix - Chris say hi etc.

) I wouldn't be at all surprised that it being a 3:1 system half pre-rigged would be one of the major selling points for them doing it this way, means they can get an the haul before the poor punter even has a chance to ponder how to do the move....
@Fultonius & Johnny Brown
The only point of climbing with a thin tag-line is that it works as a haul line so you do not have to climb with a back pack. It is a lot harder to follow a vertical limestone pitch climbing with a light back pack than to lead the same pitch without. On complex steep terrain I climb with doubles + rad line, then the rappelling is easier of course.
I'm slightly more bought into a radline or anything that can be abseiled on rather than a pure tag - for me I'd probably go 7mm or even 8mm in my ideal hauling setup - hauling any weight on a super skinny line is also just horrible. I'm not sure I've ever actually failed on a route because my ropes have been too heavy, but I did nearly fail on the top crack of Sunshine crack because I didn't have half ropes and the rope drag was chronic...
Either way... the most important thing is having a system you and your partner both a familiar with and don't hate... Most of the rope system failures I've experienced or know of from friends were trying out something new on a big route.