I enjoy playing the COD games casually. The keyword being casually. I wouldn't bother paying a monthly subscription to play it casually.
The only saving grace would be if the standard of MP improved dramatically i.e we pay them, they develop the MP, add more content, free maps,dedicated servers, being able to choose favourite maps/de-select maps you don't want to play etc.
Only then could it be argued worthy to pay a subscription.
However, what I said should be a ruddy feckin' standard in a MP game!
Most PC FPS' have that flexibility and development into them already and it doesn't require a subscription.