That's like saying without Silk Road, BTC is useless. Potcoin will always have a place in the medical/recreational marijuana economy as long as people make sure it has a place. Legal or not, Potcoin will have it's uses.
For instance... I can't buy MMJ with Potcoin yet, but I can sure as hell buy vapes and such. Vapes aren't illegal, and yet I would use my coin to buy them. I could use my credit card or cash when I order from places, but I'd rather use crypto. I'm sure there are a lot of other people out there like me that think the same way. As such, it's safe to assume that there are people out there that would rather pay for medical/recreational marijuana, and related goods, with Potcoin. It's not just about banking, mate. It's about a community coin that works for the community's wants and needs.
Potcoin not only must compete against cash,credit cards,mobile payments etc., it also competes against other cryptocoins. I think making analogies between potcoin and bitcoin is flawed because bitcoin has many more supporters than potcoin. Non-miners need a strong reason to use a niche cryptocoin such as potcoin. Miners will use whatever coin they mine.
For example, bitpay.com can be used to accept bitcoin payments on a website. But maybe they don't permit marijuana sales?
Someone could make a potcoin version of bitpay which would be exclusively for the marijuana industry.
This is just an example of how potcoin can, in some instances, be more useful than bitcoin.
I run some websites which process credit card payments to sell stuff. Few months ago, I added the option to pay with BTC/LTC but it's an off-site option. The buyer must send BTC/LTC (via wallet) and then email me. Even though, so far, no one has paid me by cryptocoin, I still leave that payment option on my sites as a way to promote cryptocoins. From my personal experience, most people still have little interest/experience in paying with cryptocoins. This will hopefully change as people are more often exposed to cryptocoins.
Perhaps marijuana buyers are different than the average person. Maybe they don't want a marijuana related purchase to be permanently recorded on their credit card statement which might embarrass them or negatively affect them in the future. Currently, I am mining potcoin and of course, anyone who mines potcoin would like to be able to buy stuff with our potcoins. Just keep promoting these coins and eventually buyers/sellers will often use them .... I hope.
In case some of you don't know, banks are important for almost every business because credit/debit card payments are all processed through banks. Without a bank account, you can not put cash in a bank and you also cant process credit/debit cards either. You can process credit/debit payments through someone else's bank account (for example: paypal), but you wont be able to open your own credit card merchant account. Almost forgot to mention, having a bank account can make it much easier to convert cryptocoin into fiat money.