back in the far mists of time, when I knew less than nothing about anything, I naively loaded up a bike (that was almost new) with lots of clobber and set off touring, with a like minded chum. We just rode to Newhaven, got the ferry, and headed off. A few days later we were in Switzerland, and my rear wheel was already breaking spokes. I had taken a few spare spokes, almost run out of the things, and at the rate I was going I would need a lot more before the trip was through.
I had already built a few wheels and I had stress-relieved those because that was recommended as 'standard practice'. Those wheels had worked fine. I'd bought a new bike shortly before this trip because my old touring bike was basically falling apart; it'd cope with day trips but further than that I wasn't so sure about it. Call me slow on the uptake but it hadn't occurred to me that anyone might sell a touring bike with wheels that hadn't been stress-relieved properly, so I didn't stress-relieve my nice new wheels before I went...
Colour me surprised when the spokes started breaking.... I remember sitting outside my tent one evening, practically oblivious to the balmy weather, the fantastic scenery etc, because I wrestling with wire spaghetti, my mind a fog of annoyance at the state of my rear wheel, and wondering if it would spoil the rest of the trip or indeed have me off the bike at some point.
I came to the conclusion that either
a) the spokes were badly manufactured and would break no matter what I did (something I'd already encountered) or
b) the wheel hadn't been correctly stress-relieved and doing a stress-relief treatment now might well do some good and probably wouldn't do any harm.
So I set to, and stress-relieved the wheel. I broke no more spokes on that trip, which included several thousand miles of loaded running, much of which was in the mountains. The conclusion I came to was that perhaps only the spokes that broke were faulty, or the stress relief treatment had been effective. Now, I am pretty sure that the latter applies. Note that the wheel remained rideable even with one broken spoke, so I wasn't greatly inconvenienced by the breakages, just worried/annoyed by them.
In the intervening years since then I have seen a lot more wheels do odd things and I have a few more ideas about how and why wheels fail or start to give trouble. It is (of course) nowhere near as simple as 'lots of spokes = good, fewer spokes = bad' but there is almost no penalty (with any given rim etc) in having a few more spokes in the wheel and it brings several advantages.
If you use a rim that is radially stiff enough, you can run with a reduced spoke count and the normal fatigue stresses that the spokes see can still be acceptable. However that a) doesn't guarantee that the rim is stiff enough laterally/torsionally to leave the wheel rideable should a spoke break (and if you do ride such a wheel with a broken spoke, the rim can take a set and wreck the wheel) , b) it doesn't mean the wheel is going to be stiff enough laterally, and c) it doesn't mean that you are any faster in the meantime, for having fewer spokes. So any wheel can be 'strong enough' for a particular task but that doesn't mean it will really be 'durable enough' for that service. A wheel that has a reduced spoke count (for any given rim/loading) sees considerably higher stresses, so why not have a few more spokes in it?
Also, a wheel that is doomed to break spokes (by fatigue) won't do so immediately; if it takes several hundred miles of heavily loaded touring to expose such weakness in wheels, this might translate to several thousand miles of unladen riding before the same things start to be seen. [A really good set of wheels ought to last tens of thousands of miles of unladen riding without breaking spokes, and at least many thousands of miles when loaded.]
So my concern is that a set of wheels that has been OK for a while when used unladen won't necessarily prove to be as reliable when used with a load on. If I broke a few spokes on the way to PBP, I know darned well I wouldn't enjoy the event itself in the same way as I would having not broken spokes previously, whether I broke any more during the event or not.
Knowing what I know now, I'd happily ride a pretty light wheel (the right ~480g rim with 36 DB spokes in it) with a 10-15 kg load sat on it, provided it had been properly built and stress-relieved. There are lots of other wheels that I wouldn't be so confident in, even if they were heavier.
cheers
