What I meant by "self-insure unless you have some special reason to think you might need it" is that insurers are the business of making money. There's nothing wrong with that in and of itself as long as they honor the deal you strike with them. I'm talking trip insurance here.
Travel insurance has a high likelihood of claims for an insurer as opposed to, say, homeowners. If you are insuring a 10K trip and want to be "fully" covered (meaning full or close to full reimbursement for losses triggered by "normal" occurances - not just something like an asteroid strike), you should expect a significant premium. If you're getting trip insurance on a significant trip and not paying a lot for it, then you're probably not getting coverage for much.
That's only part of why I say it isn't usually worth it. Insurance is like betting at a casino - their actuaries have figured it out and the odds are on the house. If you can stomach the loss, you're better off going bare and saving insurance for catastrophic losses that you couldn't stomach. It would stink to have to cancel a trip, but it won't break me. My house burning down, different story - that, I'll insure against. Life insurance to take care of my family, yes. Those are things where I'm "risk averse." Stuff that I can handle without insuring, I don't insure because I'm just betting against the house and lose in the long run. Making it concrete, I've saved way more than the cost of my next trip by not buying trip insurance on my last 20.
DAN dive insurance? Yes, every time. That's worth every penny. The DAN travel insurance, which when I priced it for 8K in coverage for the trip and the included medical was something like $600. That I'll do without.