I just subscribe once every so often to get a quick update then cancel.
For me, it seems as though you can just use the Navblue data that will be updated by MS regularly (I think), though I'm not sure charts will be a feature at launch and may be sold separately on the market. If that is the case, and the price is reasonable, I don't see a need for Navigraph.
The only other point of concern, is that if PMDG only solely allow for Navigraph data on their planes - in terms of what nav data they might use on the tablet in the cockpit. Hopefully there isn't some awkward restriction that means it won't integrate with the navblue data, though I'd be surprised if that were the case.