You do use Protect after Curse, you simply do not use Curse raw on their switch. Odds are they're going to switch into a counter that will force your own Trevenant out, or kill it if it stays in. If you Destiny Bond on the turn they come in then it achieves the same effect as Substitute, they effectively can not attack you for that turn, only without the cost of 25% of your hp. On the following turn you may then Curse, and Protect the turn after that to have, over all, more HP saved than you otherwise would with a sub.
And the point of the set is not to stall, it is to more efficiently force switches and spread burn when applicable.
I do not think Protect is useless by any means, but it is a weak play against something directly after Cursing it on the switch. They have no real reason to stay in, and Trev has lost 25% and possibly its berry while the opponent has only lost 25% and likely regained momentum since you have done nothing on your followup turn Protecting. The only reason to Protect after a Curse is if your opponent can VoltSwitch out, in order to keep them in play and incur more, free, residual damage; or if the counter they switched into play, such as Heatran, is capable of 1shotting you from 75% and is faster, as they will likely stay in play regardless so the free turn of them switching out is moot the vast majority of the time.
If neither of those two circumstances are met, after a Curse you should always be prepping a Will-o-Wisp or new Destiny Bond to force the same catch 22 on the new switchin.