It seems that the linking will occur with the start of the season. In reality it makes sense on several levels. Several years ago, Roku did not have spring training games. I would guess that next year, in year 2 of the new system, spring training will be included. Just my WAG on that. The March 31 comes from multiple posts on DBS sites.
As a standalone MLB.tv customer I don't think that's right. Mine renewed a few days before the first spring training game this year. It says it's good through next offseason. I have been a customer for a few years now and I always have to pay before spring training to get access to spring training games. The reason roku didn't work for spring training last year is because they didn't update the app to include those games in the schedule. PC, smartphones, and tablets worked for spring training last year.
Luckily that isn't an issue this year. Every device I have tried has already been updated with this years schedule. I have watched several spring training games on Apple TV and Roku already.
What you will get is access to archived games until spring training starts next year. It's a shame they aren't giving you guys access right now. You already get the regular season through EI on your TV. EI doesn't include spring training games so one of the benefits to them packing in MLB.tv should be access to all the televised spring training games with no blackouts.
EDIT: The included gameday audio feature is pretty great too. I listen to all the Tigers day games using the At Bat app on my iPhone when I'm at work. We don't get a good radio signal from their channel in our office building so it really comes in handy for me. I'm listening to my favorite radio announcers for the Tigers vs. Blue Jays game right now.