As @dragonwriter said, the HSA doesn't protect you against ruin, the linked high-deductible insurance does. You use the HSA for your ordinary medical expenses and get a nice tax discount from it. The HSA is also acts as a retirement investment account.
So, if you're young with minimal healthcare needs, you can get multiple benefits:
1) High-deductible insurance that's (relatively) cheap but still protects you against financial ruin
2) After 10 years of maximum contributions, you'll have ~$10K in cash for emergencies and about $33K in retirement investment contributions.