I pay with BTC regularly, using it on several sites. The devaluing of the currency and 'holding' assumes that no further income in cryptocoins is being made..
There are the miners, and there are the speculator investors. Essentially, the coins are volatile around a generally stable mean, which as you point out has been devaluing over the year. However smart trading in the ups and downs in the generally stable periods cann still make a lot of coin on the markets.
Then you spend it in places like scan.co.uk, buying computer parts. Or trade it out to your bank account through Coinbase or some other site. I used to spend it on steam, before they cancelled it when the network fee jumped too high. At one stage it was around £20 per transaction.