If you do, how do you figure out if the investment appreciated or depreciated?
I mean, if I use BTC as a currency to buy things, I don't understand how I would evaluate every purchase at the end of the year to figure out whether I owe capital gains.
It's certainly possible - one way would be to do like some people who do some casual stock trading where you record all the transactions in an excel sheet with the amounts and dates, and then submit the appropriate reporting to IRS. Similar measures apply if you do significant amounts of barter; e.g. if you as a farmer trade a truckload of produce for an old vehicle without any cash involved, then you may have (all kinds of legal caveats apply) to appraise the transaction at some dollar value and justify that appraisal. For bitcoin it's a bit simpler than barter since just taking daily reference prices from a major exchange is an objectively reasonable method of pricing.
It's not that hard, but takes some work and skill - it may be prudent to hire a certified accountant to do that for you properly.
In any case, the fact that in some circumstances the reporting is labor-intensive, or expensive, or unclear isn't a valid excuse to not do it. You can ask IRS or a CPA for assistance, and if you do your best then even if IRS eventually disagrees with something, then it's going to just be a correction (which may go in both directions).
You can imagine you are converting BTC to dollars right before purchase. If you had 1 BTC worth $1000 at the beginning of the year and then 6 months later (when it was worth $1500) bought something with it - you earned $500 in capital gains. (Not saying this is the current law, but that method seems to make sense)
If someone tells you to walk 1,000 miles to get somewhere you need to be, would you really respond with anything except "I can't"? It's an acceptable shortening of "well that's a really long way, and it's really hot, and it would take a long time, and I don't have that kind of time, and I'm not in very good shape, and my knees are bad"
Sure you can, you just have to convince someone it's worth the hassle. That it is atypical doesn't mean you can't.