This is a bad reason to invest in mining hardware. If you expect bitcoins to appreciate, you buy bitcoins and either hold them or put them in low-risk investments.

If a mining investment (or any other investment) today costs 1 BTC, then it better be worth more than 1 BTC (profits paid out + resale value) at some later time, or it was a bad investment.

This doesn't take away the fact that there are plenty of people buying mining hardware using the same exact reasoning ("if BTC appreciates i'll make a profit, despite difficulty increases") and that's one of the reasons why mining is unprofitable for small-scale miners: There are too many people willing to mine at a loss.

USB miners are novelty items that can be fun to have and use, but not investment opportunities. But not every hobby needs to make you money.