Its a good idea, but the thing is if anyone wanted to invest in crypto, why would they buy the services of coinflash, and go through the whole roundup thing and than buy through our own coinbase account. Its just our money they are giving back in form of cryptocurrency.
Instead that investor will directly go on and buy by himself any time. But this idea will be useful if it offers any cashback reward or anything else. Suppose the roundup for a month was 10$ so than maybe offer 0.1-0.5$ over that and buy BTC worth of 10.1-10.5$. It will attract more user this way.
Sorry if I am wrong but its just my thinking.