Speaking from anecdotal experience, this is not true. As an example, I heard some employees from the company I worked at left to start a company to make messaging software. They ended up with a half-broken IRC client with a terrible UI and no features. They then sold it to the bank for millions of dollars.
If it's expensive, banks like it. Quality is a plus, but not required.
It was my impression when I was interning at Citi (quant/strategy/trading) that they weren't very experimental buyers, and frequently liked to reinvent the wheel. They certainly didn't redo EVERYTHING. Besides that, there aren't that many big banks to sell to. I'd say it's trickier than average to get a market fit. But maybe you're right that they'll buy any old crap.
And surely having worked at a bank is a huge advantage. Especially if you go in there with the intention of finding some area to help them out with, and network like hell.
If it's expensive, banks like it. Quality is a plus, but not required.