Just thought I'd chime in belatedly, as a passionate long-hose diver.
There are two things:
primary donation vs. secondary donation
and
How long is the hose on the reg I donate
Donating my Secondary:
+ No need for me to swap regs
- donating a potentially silted up reg
- having to find said reg (where is it clipped, is it still clipped?)
- finding a way to reliably clip the sodding thing to myself (haven't found a way, but magnetic clips seem to work for others).
Whereas donating my Primary (the one I am breathing out of until just now):
+ Known Working Regulator given to diver with the most pressing need of gas
+ Known position of regulator for me & buddy (most people breathe through their face while diving, after all...)
+ necklaced regulator is easy to find for me
+ necklace (Bungee-cord+two cable ties) works for retention, can be pulled apart if needed and costs naff all.
- I have to be able to swap regulators on short notice
So, for me, the question of donating my primary compared to my secondary is a moot point. You get the reg I've been breathing out of (if you haven't already helped yourself). If you insist on grabbing my secondary, feel free, I'll cope. As a thought: If you cannot cope with having your reg taken from you at random times, how are you going to cope if someone kicks it out of your face through inconsiderate finning at inopportune times?
Now, how long is my donated hose?
Short:
+ Easy stowing of hose
+ diver near me through design
- Me & other diver are effectively entangled
Long:
+ can give other diver time and space to sort problems out
+ can swim easily side by side
- must stow hose somehow (unless doing the "DIR-Wraparound")
- must consider controlling distance
Now, if I donate my primary (see above), then I can deal with the stowage issue, by doing the "DIR-Wraparound", turning my primary 1st stage to send the 210cm/7' hose downwards, tucking it in under my BCD (I dive with a Seaquest ProQD, tucking the hose in underneath works well without needing to spend money on a lamp), bringing it around my gut, over my left shoulder and around my head with a swivel (90° elbow would do just as well, though) to feed me from the right. No, I don't wear a snorkel, that's what god gave you BCD pockets (or, French CMAS only: Knife-holding bungees on your ankle) for. For stowing my rig on the surface, a bolt-snap tied to the hose allows me to clip my primary to the necklace of my secondary and a random d-ring on my BCD to stop both 2nd stages dragging. The relatively stiff hose doesn't snag that easily. An additional advantage of the 'round the neck' thing is that you have ZERO pull on your reg, however you turn your head, as there is enough slack in the hose. On a short primary, usually, you either have restricted turning to your left (the hose runs out eventually, if not set up right), or you have a huge loop of hose to catch off your shoulder (if it's long enough and not hi-flex hose).
So, if you are diving with me, I'll be giving you my Abyss/MR22, and you can sort yourself out, I'll breath my Proton ICE/V32 in the meantime.
This is, incidentally, explained to the CMAS* course candidates here in pretty much the same way, and *everyone* I have lent my regs to (Sherwoods - what we have as club loaner regs - are good, always give you air, but some people just don't like breathing them, and if I have a cold and can't dive, I'll lend anything of mine that works to anyone who asks) has spent the €35/$50 extra a long hose and a swivel for the primary. I wonder why.
As an aside: single first stages effectively expire your insurance diving in cold water in Germany. Even in summer, the thermocline at 20m / 66' will bring you in contact with sub 10°C (50°F) water. You can dive with single 1st stages, but why do you want to be ridiculed and ostracised for an extra €109 ($158), which is the price of a plain DS4 around here, I don't know, especially with other risks involved. With two 1st stages and routing the primary downwards, stowing the hose is no issue.
Valve drills for turning your own regs off are a different kettle of fish altogether... And to reassure the purists - my Proton Ice necklaced secondary has no swivel or similar excessive failure points; it is my backup, after all.
Gerbs