I have no qualms about trusting Tether for something like accepting it as payment method for a P2P trade. I don't hold that Tether for long ,... and will transfer that to my exchange, or use it with a swap service, or whatever, and either move that value into fiat or use it to buy bitcoin.

But using Tether as some type of store of value, makes no sense to me, when I have a bank (for fiat). I can see, however, how those without access to a bank (e.g., in developing countries), Tether provides a dollar-stable token that us useful as both a payment method and a store of value. They are trusting it because their other options are worse.

As far as the Tether currency stock being so large (e.g., more than 10% of the value of bitcoin), it's unlikely Tether is going to be dumping bitcoin (like Tarra ponzi reportedly did) to support their peg. I don't even know if they have bitcoin in their reserves (nobody knows, they don't share publicly the components of their reserves).