While they give you a bog standard Suica card with Y1500 credit, the N'Ex ticket you get is for a specific seat on a specific train on a specific day. I don't think that you want to commit to your plane not being late, immigration not being slow, etc., weeks in advance.

Also, if you buy a ticket from Narita airport to Yugawara, the savings will be somewhat variable (maybe a slightly better deal, maybe a slightly worse deal) compared to just using it into Tokyo. This is because of how Japanese train fares are constructed, there is a slight saving in doing A-B-C on express trains without stopover compared to A-B and B-C ticketed separately.