You don't need to update the maps every year, but it's nice to be able to have them up to date, especially if you're travelling to areas you're unfamiliar with often. Even at £70, it's still expensive compared to a one off payment around that with updates for life that you can move between cars. I can't get maps for that car from BMW either, so I'd be stuck paying £171.
What car do you have then? Granted, I haven't asked about maps for my NBT 530d yet, but my CIC 335d the maps were only £25 or something. You can buy the DVD's and upload them to the cars HDD, then you just need the FSC code to activate them.
Alternatively you can just download the maps yourself and buy the FSC code. But they are over 30 GB IIRC.
But I get your point, not as cheap as free lifetime updates with a TomTom or similar. But you only really need to update once in an ownership life-cycle really. If at all. Most roads don't change. Although if I were planning a big road trip across Europe I'd probably consider an update first.