It will be useful & functional once the security and scalability is at its best. Minimal transaction times and costs is why cryptos are so special and interesting to use as a currency. This probably is the most important usecase for the whole technology.
I have got fed of of hearing the same things about bitcoins which are not true at all. You mentioned that bitcoins has minimal transaction time and low costs which is not true at all. If you have expereince of other payment options like PayPal, Stripe, Credit Crads, bank transfers all these are way faster in comparison to bitcoins. In case of bitcoins one has to still wait for 10-20 minutes (as per current situation) for a a transaction to go through. Most of the online deals and payments are done in seconds and thus bitcoin is impractical for those transactions.
It's not about Bitcoin, it's about cryptocurrencies and its technology in general. And no, bank transfers are way slower, even than Bitcoin.
Probably one of the fastest with minimal costs currently is Stellar Lumens.