By this logic, Coinbase basically can't win. If they do what they need to do to allow Bitcoin to become popular -- that is, make it as pleasant to use as a bank with credit and debit cards -- they essentially become a bank, thus earning the scorn of the Bitcoin community. But if they don't, Bitcoin remains a niche product that no one besides a few ideological supporters with axes to grind will ever use.
This is a false dichotomy. Bitcoin is a useful protocol for other things besides consumer payments. It is also possible that even if Coinbase's value proposition is 99% as an exchange, there may still be some value to its users in providing a convenient device for extracting $ from their btc balances. Someday I could see Coinbase as a sort of wrapper around multi-currency wallets. Users may have btc, ltc, $, and various other digital assets which they (stupidly or not) wish to leave in one repository.
