Everyone's usage is different, but people do generally fall into one of two types.
First is the type who, once they have enough storage for the OS, regularly used apps, and perhaps sufficient media/games to provide for a couple of days consumption, don't normally fill the rest of it just because the space is there. The extra room going to a larger capacity model above the one that does suit their needs is seen as, and would be, purely a luxury, not a necessity.
Second is the sort who, after having enough storage for the OS, regularly used apps, and perhaps sufficient media/games to provide for a couple of days consumption, will throw on media/games they might like to listen/view/play at some point, and will migrate towards the largest available version out of concern the 32GB would not be enough. The reality is, though, that for most of them, going to the larger capacity will only provide a bigger closet that they will be as likely to fill and find themselves out of room.
Now I don't want it to appear I'm knocking the latter - I tend to be of that type myself. But recognizing that about myself, I opted for the 32GB, knowing I could manage with its storage, because I could actually lay hands on one much more easily and sooner than on the 64GB model.
I suspect nobody who buys a 64GB model over the 32GB will regret it - at only $50 for the upgrade and offering more than twice the available storage, it's not a bad deal at all for the convenience it provides. But by the same token, I suspect that many who, like me, purchased the 32GB simply based on the elusiveness of the 64GB, are not feeling as limited by it as we might have initially feared.
I agree for the most part. But there are certainly more than two types of usage cases. Another one, for example, would be someone who has plenty of space leftover after OS, apps, and media/games consumption but then takes on a new career/hobby that changes their usage in either direction.
I don't think either model is a bad choice. For my own personal preferences, I went with 64 GB for the extra closet space because I maxed out the 32 GB on my Moto X in less than a year without even trying. But without going into details, I would have to assume that my usage is much higher than the average person. But that's also why it's my personal suggestion for anyone to just spend the extra $50 (a small percentage of the overall device price) to double the storage. Gone are the days where we can say "whoops, I'm running low. Let me jump on Amazon and get a new micro-SD".