BF3 is just too shoddy right now to introduce a subscription model. Bugs, imbalances, missing features, various scandals around hacking, the community having to threaten EA before they'd open up about DLC, etc.
But, somewhat sadly, I see sub fee's in future BF titles as an inevitability. Business is about profit, nothing else, so the greedy money men at EA have seen whats going on with CoD and immediately shot their metaphorical load all over their spread sheets. MOAR!
That's not to say its all bad though, they could pull something really special off. Perhaps a BF interpretation of Planetside, fighting over maps that are several hundred KM in size. Or even just a greatly improved version of what they're doing now, with regular updates and proper support for competitive e-sports. Who knows.