Once a 2nd major exchange enables LN deposits & withdrawals, all of them will be essentially forced to do it so as not to lose out in the arbitrage battle.
Arbitrage is actually really an excellent point - this could perhaps even be Lightning's killer app, as payments in BTC (be it per LN or not) are still not too popular, but the speculation business with BTC is thriving as never.
So maybe Bitstamp customers should spam them with feature requests to try to get it added ASAP? If they already operate a node (and have already more than a year of experience with it), it should not be too difficult for them.
However questionable Tether is as a concept, that would be a pretty remarkable facility for Bitfinex traders to have at their disposal. I thought that Tether had changed into an Ethereum based token though?
Tether currently operates on several blockchains. Bitcoin (via Omni and also on the Liquid sidechain) continues to be one of them, but they added Ethereum, EOS and also some minor altcoins like Tron, SLP, OMG, Algorand and Nano. See
https://tether.to/faqs/ .
Tether could also be huge for LN, because once it's possible to transfer an "USD token" around without high delays, the arbitrage business would get a new thing to play with. And it would also perhaps mean that the "minor altcoin" incarnations of Tether would slowly die because they wouldn't offer more transaction speed than the Bitcoin-based TUSD.
But as far as I understand the concept of "tokens on LN", Tether channels would not work for Bitcoin and vice versa. (There could actually exist some magic tech I am not aware of, but how should the HTLC concept work for different tokens with different values if the tokens aren't locked each one separately?) Anyway, it would bring increased publicity for LN. I also do not really like Tether but if it helps ...