It would be nice if in the design we could put text or images that won't actually show up on the backend proof. Example of needing this could be a postcard that says:
Hi First Name,
If you're interested in selling your home contact me today....and so on...
Where the "hi first name" is going to be different on every postcard as it's variable data so you wouldn't want it on the actual proof.
Don't know what software you're using to produce, but you should be able to instruct your customers to put placeholder text that you can replace with the correct tags to create the merged document. I'd find the ability to place non-printing images useful for instructions or guides with a little more explanation, but I don't really understand the purpose of non-printing placeholders.