I can't speak for the ENC guys but from a Canadian perspective I'd rather play lower ranked sides than face a Tier One sides "B" team. I was glad to see the Churchill Cup ended for this reason, it served it's purpose when Canada/U.S.A. were struggling from 2004-09ish and was kind of a waste of time after that.
It's tough. There is such a dropoff in quality in the ENC currently after Georgia and Romania that it would not be particularly beneficial to play these other sides. Russia and Spain at a push might be OK, but they are still well below the necessary standard both on and off the pitch. Not much point in a 4 team tournament either when we already play each other almost every year. Here's how I see the viability of the other unions in the ENC:
Russia: terrible coach and terrible union. Weather is an issue for them in the Feb-Mar window so they either need to find reasonable venues unlike Sochi and their most recent disaster, or they should go try and join the Asia 5 Nations which is played in a better time of year.
Spain: mediocre union, doesn't really seem to be going anywhere on or off the pitch
Portugal: seemingly disinterested in professionalism so you can pretty much dismiss them until that attitude changes
Belgium: going back down to 1B but they seem to have a bright future on and off the pitch. A potential candidate for inclusion, but only probably 5 years down the road.
Others:
Moldova: maybe they can turn into the new Georgia but they are a long way away from that happening
Germany: huge country and economy, but lots of competing sports make it tough to see rugby making much headway
Netherlands: has dropped off over the past 10 years but they have announced an ambitious program. Long term candidate.
I'd go with England Saxons and Ireland Wolfhounds until others are up to standard on and off the pitch.