While I agree that toogle shouldn't need two slots, mend wound is the second "active" toogle and the first who can work with a bar swap (the first being overload). When you toogle on mend wound, you make a tradeoff : in exchange of your new healing skills, you can't light/heavy attack an enemy, so no free damage nor resource recovery with heavy. Other toogle are just passive you can forget, there is no downside to slot and active on your back bar if it work on your front.

I won't be again a single slotted toogle mechanism (I keep asking it for pets especially), but mend wound is not exactly the same as the other, so I can understand the logic behind. In my opinion, the ideal solution would be :
- for active toogle (mend wound) : remain active once used even if bar swapping ;
- for pets : either remain active but without the ability to fire their skills (sorc pets) or loose a lot of damage (warden bear), or vanish temporary when bar switching on a non slotted bar then respawn immediately when switching back (allow the user to save it from one shot mechanism) ;
- for other passive toogle (such as old FoO, inner light, daedric armor) : continue to change them to interesting skill while keeping the passive effect (even if I would love to see pure passive effect disappears, but that would need a huge rebalancing).