Only being able to use "Primary" Addressee/Salutations in NXT email is not a solution (I mean, who ever addresses their emails with an Addressee, which is meant for the front of an envelope?).
We want the same functionality as Online Express had, where you had the choice of primary, secondary and tertiary addressee/salutations. We defined one of our salutations as the Constituent ID, and used this in emails where it was important to be able to communicate the constituent's ID to them (for example, filling in surveys so that we had a guaranteed match back to our data etc).
The ability to pull in Constituent ID is a key piece of us being able to use RE NXT emails for all our emails. We have a number of surveys that rely on a person putting in their constituent id before they can complete the survey. We also run an election every three years where alumni have to enter their constituent ID before they can vote. Please add constituent ID as a merge field.