Like you Lawrence, I like walking the cup with my so-called weak hand especially with a mirror because you have more control than mirror welding free-hand on pipe although, I've had to resort to both in the past.
Nowadays most of the pipe I weld is schedule 10 & rarely schedule 40 316Ti or 316L and schedule 40 Grade 2 Ti... On the butt joints (groove welds) I prefer walking the cup as well as when I'm welding the slip-on or ANSI flanges... The branch fillets (no weld-o-lets here) on the smaller ( 2" or less I.D.) schedule 10 diameter pipe has such a fit-up due to the hole diameters we drill enable us to contour the branch fitting so that the I.D. wall mates with the appropriate hole wall leaving plenty of "meat" where the outside fillet weld is to be deposited & still have CJP... As a result, my free-hand welds are superior to walking the cup & HI is much less (we've got a real nice heat sink set up that virtually eliminates any type of movement or distortion but, my company wo'nt let me share the details!) + they're completed in much less time so long as I've got the parameters set just right so that I can "fly" around the joint with my pedal to the metal and I remember that "once I'm halfway around the track so to speak, I can start to let off the gas and get ready to coast a bit until I get towards where I started!"
Also, once I'm over the place where I started - I slowly ramp down the current myself with the (gotta love a foot control or a torch mounted amptrol!!!) foot control which is hard to do if one is welding superheater tubes in a boiler because, "They Aint no foot pedals in a boiler house Son!!!" "You've got to use yo eyes, yo head & hands Son!!!" This is where walking the cup really helps and you can "move like the dickens" around pipe if you focus on what going on where you're welding... Now an on-off switch connected to a power source with a downslope control is a pretty good option to have in a boiler house as most tube welders will attest, but that's still was considered a rare luxury when I was a tube welder back in the day!
Bottom line is, most pipe joints can be welded either way but "walking the cup" gives you better puddle/penetration control in most situations although there are some situations whereby free-hand welding with GTA results in superior deposits than one would have with WTC! This has been my experience but, others may disagree -big deal!!! Especially if the HI is within specs, every other parameter is followed and the weld pases NDT!!! NEXT!!!
Respectfully,
Henry