Versatility. You are going to struggle boiling water over an open fire with 95 pat water bottler and mug. The attachments for nalgene bottles make this very versatile. I love the coffee filter idea for either use as a water pre-filter or as a coffe filter. Great idea. I wonder also if this could be used "kelly kettle style" with a hobo-stove setup?
Rubbish! It's simple, you just don't create a blazing fire and make a small brew fire. A few small nails make a good trivet to put you mug on and a small star fire underneath will be adequate to boil water. I start to get the feeling you haven't tried this at all and yet post on things as if you are the leading authority! Come on Martyn, people have been using metal mugs for years, if they didn't work then they would have figured that out by now!
On the other hand, the Olicamp style stainless mugs can be drunk out of without your lip being severely burnt, but the black metal mug posted above is a nightmare without the plastic lip guard which often gets melted as it is easily forgotten about. Also, the lip guard isn't a perfect seal around the mug and I found that a small trickle of tea would pour out underneath the seal and drip all over my shirt!
No issues like that with the newer Crusader mug though. The black NATO bottle is a brilliant design but some people cannot get over the plastic taste. The metal bottle won't suffer from this, but I do have a stainless bottle made for US Rangers which I expect this model of bottle is based on, and it does rust! Somehow, I can't think that would be good for the digestion so don't bother using it and have gone back to plastic Nalgene and NATO bottles.
Those add ons are really good though, inspirational.