Ken Doctor has a very smart and interesting take on the news that the NYT now has 390,000 paying digital subscribers — plus another 16,000 at the Boston Globe. It’s unambiguously good news, on many fronts.

First, and most importantly, digital ad revenues went up by 10% in the area of the business with the paywall, while plunging by 26% at About Group, which doesn’t have one. The big worry about the paywall was always that it would eat into ad revenues, and that really doesn’t seem to have happened. Of course, it’s impossible to know what the NYT’s digital ad revenues would have done sans paywall. But my gut feeling is that it’s a net positive: it allows for much more targeted advertising and therefore higher ad rates.

What’s more, the NYT still has massive reach outside the paywall: it has at least an order of magnitude more unique visitors each month than it has paying subscribers. The NYT can still sell those other visitors just as it always could; they certainly haven’t become less valuable since the paywall went up.

The only possible cloud in this picture is in overall traffic growth: the NYT doesn’t give pageview numbers, but sites like Quantcast and Compete say that they see no real growth in traffic to nytimes.com, and possibly a small decline. Again, the counterfactual is impossible to know: would traffic have been bigger had the paywall not been in place? I don’t think that the paywall has reduced traffic very much, but I do think that the amount of time and money and editorial effort which went in to constructing the paywall might well have found its way into other innovations, had the paywall not happened, which would have made the NYT an even better and more popular product.

That said, the paywall has probably paid for itself already, and with luck some of the extra cashflow it throws off will be reinvested in more consumer-friendly innovations.

The other big news today is this:

Churn is less with digital than print customers: Skeptics opined that people might sign up, but then flee after sampling the paid digital product. The opposite appears true: Smurl says digital churn is less than print churn.

I didn’t expect this, but I believe it, and it’s really great for the NYT. It’s easy to cancel a NYT subscription, but by the same token it’s easy to keep one, too. And it seems that once you’ve taken the plunge and started paying for the NYT, you keep on paying — even more than with a print newspaper.

The result is that the NYT’s digital subscribers are a bit like a bank’s depositor base: although in theory they could leave at any time, in practice they’re an incredibly stable funding source. Much more stable, to be sure, than any advertiser.

But while I’m happy about this state of affairs, I still don’t really understand it. Here’s Doctor, again:

It took about 12 seconds for Times’ readers to figure out the new subscription math, when the company when digital-paid last year. When they did the math and saw they could get the four-pound Sunday paper and “all-digital-access” for $60 less than “all-digital-access” by itself, they took the newsprint. Which stabilized Sunday sales, and the Sunday ad base. Then the Times was able to announce a near-historic fact in October: Sunday home delivery subscriptions had actually increased year-over-year, a positive point in an industry used to parsing negatives. Now, Sunday is emerging a key point of strategic planning.

This is great news for the Sunday newspaper, which is highly profitable for the NYT. But it also raises the obvious question: why are 390,000 NYT readers eschewing a Sunday paper they could get for less than nothing? Some are IHT subscribers who don’t have that option; others are naturally peripatetic. And the cheapest digital subscription is actually still cheaper than the Sunday-only delivery.

It seemed to me, when I entered into my ill-fated bet with John Gapper, that NYT readers would go for the free access bundled with the paper, rather than plump for digital-only access. But increasingly it seems that readers actively dislike having to manage a physical paper, and are willing to pay for making the whole experience virtual.

If that’s the case, then the least the NYT can do is to continue to invest in its iPad app. Right now the website is still superior to the app, except for offline reading. The app desperately needs search, and it needs to retain hyperlinks from the original articles, and it needs to somehow build in the sense of serendipity and of relative importance which newspaper readers love so much. It’s hard to tell what’s important, in the app, once you move off the front page. And it’s hard to have your eye caught by a great story you didn’t know you wanted to read. But those things will come, I’m sure. If only because there’s now a very healthy income stream — Doctor estimates it at more than $80 million per year — which can pay to help develop them.

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Felix Salmon is an Audit contributor. He's also the finance blogger for Reuters; this post can also be found at Reuters.com.