actually the yearly membership is 52 skirmishes, as u said, while u only can play 50 skirmishes with 5k hc and not 5000 :P
100 free magna every week equals 5000 shards every week, double the amount of laddergames + 10% shards equals not 10% shards but 120% more shards^^
so while the 1 month membership brings u less skirmishes but more shards, the yearly membership brings u more skirmishes and more shards. additionally u have 2 raidslots as member instead of 1, which also counts toward the challenge arena replacement.
i think thats not bad, and u can be sure that the benefits for being member will increase with every future content :)