The more you say about this SA, the clearer it is that she's giving you horrible advice. The felt is NOT useless. You need to put back this felt over the strap, to prevent the resin from making marks on your bags; especially the resin on top of the front panel of your bag, and the the interior lining of the flap. I had bought a resale bag from eBay, and the seller made no indication of the felt being missing when the bag's in storage. I was so excited in finding the bag, I forgot to ask for interior pictures of the bag. When I received the bag, and opened it, I unknowingly peeled apart what looked like the resin having fused with the interior lining of the bag. The interior lining is now peeled and H can't do anything about it, short of replacing the interior lining.
That's my same sentiment.
When you handcarry a Birkin, the contact point is broader, as in the width of your palm. But when you hang it on a hook, it's that small area that takes the whole weight of the birkin. And the handles are strained all the time; from usage and storage.
Letting it have its beauty sleep is what I like best for Birkins that are not in use. In humid countries, make sure there's a dehumidifer. And take it out from time to time to air it and moisturise it with leather cream. Air it completely before storing it back again.
The only Birkin that I hang up, as an exception is my SB. I've given away the huge box, and it's horrible just sitting on my shelf as it is so slouchy and collapses forward. So this, I hang it up.
Boo & lovely,
Glad to be of help. The damage can be quite nasty if a bag is stored away for a long time without the felt protector. It only overs the the resin on the front panel of the bag. The sides have no protection. Which is why sometimes when you open an old H bag, the interior lining under the flap have marks on the sides. In worse cases, patches of the interior lining being peeled off!