Bancor is still under ico price, and makes 40% loss in ETH scale, which is unacceptable, although the team does the work hard, they may lack some skill to let investors make money.
If we see Bancor's value based on parity with ETH then yes, you are right. First we need to set a base currency as measure to judge the value of any project because large majority use which suits them. Fiat was used to buy Ethereum so it isn't like this in fact. Bancor is undervalued now but isn't under ICO anymore. It is different perspective of view of everyone.
If you're looking at it from an investor point of view, you need to ask, what's the alternative to the money I am investing? i.e., if you didn't make the BNT investment, what would you have done with that money? If your answer is that you would have left it in the bank, then you measure its success against USD. If your answer is that you would just have left it as ETH, then you measure its success relative to ETH.
That's what most people do. They convert a part of their ETH into tokens like BNT, so it is important for them to measure success relative to ETH and not USD.