I find it helps to consider the long view when it comes to HSA saving and investing. The money I'm stashing in to it, is for next year all the way up to age 70+ medical expenses, not those for the current year. I'm fortunate though, to make a very high salary and I've already maxed out the 401(k) (not eligible for Roth due to high income or deductible tIRAs) so we absolutely go to the max each year to reduce taxes now. We also have relatively low medical expenses in my family of 4. So we can afford to contribute the max for the year and not tap it. (Both my husband and my employer also contribute some to our respective HSAs so that is like gravy).
I can see that if you are not maxing out your other tax advantaged accounts first, it may be something that you think hard about because if you need it for non-medical expenses, there are clearly downsides to accessing the money for ineligible expenses. A Roth IRA provides you more flexibility if you need the money early for some reason (although if you have a Roth 401k like you mentioned, you may have more trouble accessing those funds readily).
However, like @60MinuteFinance, when the occasionally medical expense or Rx does come up and if you have other funds to pay for those things without it being a hardship on your regular living expenses, you can keep good records and 'reimburse' yourself for them at a later time...even in another year...even in another decade.
You didn't mentioned how much you make, but being single, young, with no kids - look at the tax advantages angle. A higher earner (like my family) will benefit the most from HSA contributions meerily because they will save more upfront on tax reductions. If you are in a lower tax bracket though, the HSA deductions are not as valuable to you but a Roth opportunity may be ideal to lock in no or low-tax investing dollars.