It might be, assuming two things:

1) That you actually care about this metric. I don't, I do not get paid by the number of minutes spent on pages, I get paid by the number of signed-up subscribers who use my software to make their workflow easier. I can (and prefer to) use bandit testing to measure the performance of redesigns.

2) that it can be reliably measured, which I don't think it can.

We are a SaaS company also, and time spent reading manual/tutorial pages is important to us.

With regards to point two, being reliably measured sounds to me like perfection is the enemy of adequate. Perhaps in low volumes you can't measure certain stats like this reliabily but in large volumes I think it's useful.

Tom Gilb once said that "everything can be measured so accurately that the result is better than having no measurement at all" or words to that effect. I'm sure he phrased it better. In this case, some users aren't measured, which drags down the accuracy of the measurement.

Even if the numbers are off by quite a few per cent, I can easily see how some site operators might benefit from knowing e.g. that visitors close one tutorial much quicker than the others.

As long as your measurement is a random sample everything is okay. Even if it is not, it is much more information than you had before. You just need to keep it in mind when evaluating conclusions. We are not talking about drug tails here and no one dies if the measurement is not 100% accurate.

I am able to measure everything 100% accurate. But this is really really expensive. It's a trade-of.

Unless your page is google.com, where more time spent on page means the experience is worse, not better.

Context is important, and it's one metric amongst others that should be used.

The point is, there are clearly actionable points GA can offer. It's disingenuous to think there are not.

I agree, saying that there are no actionable points in GA is like saying analytics in general are useless.

Counter example: I search for information on a subject. I get many interesting and relevant results and therefore spend more time on the page.

This makes it even more obvious that a longer or shorter session length is not clearly better or worse, it depends on a lot of other things.

Hook into onbeforeunload and call your own javascript to post back to your own site to let you know when they've left.

I know how I'd do this, but have no reason to do it.