Disclaimer - founder of http://capp.io and this is basically what we found in our market research.
Is it retail investors they are worried about, or is it expecting that the market won't undo any trickery?
Often, people end up being smarter than you think, when given the opportunity (not that I'm endorsing Robinhood... I'd be very cautious about using them, based on first impressions)
I would hope ETFs are available on this platform, since they trade like stocks.
As for efficient markets - I'm still trying to understand how social institutions that melt down every decade or so and are a consistent focus of criminal proceedings can be considered 'efficient.'
If "laypeople" just means "not financial industry insiders", then no, it's not necessarily bad.
If "laypeople" means "people who haven't bothered to spend any time at all learning how the market works (or at least, how it's supposed to work)", then yes, it's a very bad idea. For those people, the market may end up being functionally equivalent to a casino, but without the free drinks.
Mentioned in another reply, I would hope ETFs are available on a platform like this, since they trade like stocks. Hopefully that would allow more people to put small amounts of savings into index funds. That being said, Robinhood wouldn't be very innovative on that front, since many brokers offer the more popular index-based ETFs without fees anyway.