Premier essentially gives you the highest possible edition of the game, so DLC should come with it. If you want to play a game after the sub end, you can buy the game if you think it's worth it an continue (by which point it will a lot be a lot cheaper to buy too).
So does that give you the game and any DLC they might release?
It doesn't work if you are only interested in 1 or 2 games, but if you want access to the full catalogue and new releases Access Premier is a really good deal - all comes down to what games you want to play. Personally, I'm still on the basic one as nothing has been added to Premier to make me jump in just yet. They also offer a heavily discounted price if you pay for 12 months in advance - £89.99. Even if you only play 3 new releases there is a saving there!
Alright so. Here's my concern - what if...You decide to stop paying. You cease to have access to that game, so you're paying what? £15 a month so £180 a year to rent something. I can see that being a worthwhile proposition for fifa which is BASICALLY worthless when the new version comes out anyway - but what do you do if you decide you want to replay anthem, or mass effect etc.?
Well... I for one would not have given EA any money if it weren't for Origin Premier. I love it! Keep it up EA. I wish everyone would have the same model. My money would go bye bye!