I must admit printing vouchers is rather archaic, and while there are ways to get deals on the Nectar app, I always do my shopping with the Sainsbury's app and it doesn't (as far as I can see) allow you to apply or see what vouchers you have. I don't use the Nectar app and don't even think I know my login details.
Waitrose does let you see deals on your phone (not sure about the handheld scanners), albeit in a clumsy way where you must activate a voucher in your list.
Vouchers could be applied automatically as the scanner/app knows who you are (like BK and McDonald's shows you your points and offers on a kiosk if you can your member number) but I suppose they don't want to make it easy for you.
The Nectar log-in is now the same one as your Sainsbury's website log-in.
If you check the Nectar app you can activate both money off and extra points on certain items, these will show up in the Smartshop scan as you go app and handsets, similar to what you say Waitrose do.
They will automatically apply on the scanning handset, providing you did activate them first.
I get an extra 20-100 Nectar points a week and save money most weeks on the personalised offers even if its just 20p, its often something I am looking to buy anyway and the point offers almost always are, this week I got 10 bonus points for buying an onion, so thats 5p back on a 15p item.