As several people have mentioned, the numbers on the bottom of the check could be more than enough to steal money from your account. The trick is getting away with it. This is the problem too with credit cards. How do you translate data into money? The holy grail of course, is to get the mag-stripe and PIN from an ATM card and take cash from an automated dispenser, far from human witnesses and disguised for the cameras.
For the routing information and account, as mentioned, you can make phony cheques - these then have to be passed. Like fraudulent credit cards, one option is to buy easily fenced goods - but then there are witnesses, and trails with serial numbered goods, etc.
From my experience, I suspect most banks barely do a cursory inspection of cheques for name, signature match, etc. However, merchants that sell valuable and easily fenced goods probably have their own anti-fraud measures. The trusting days of “Catch Me If You Can” are long gone. I would also not be surprised if the US banks apply some of the same antifraud patterns to cheques that they do to credit card transactions, looking for unusual patterns.
If you create electronic transfers - well, I typically use these for more established vendors - the city for taxes, my cable company or to pay the heating bill. They want this information for regular withdrawals, and so are not the sort to monkey with accounts.
I don’t know what criteria banks use to allow account transfers, but I suspect if we can dream this up in a few minutes, banks have thought of this and have procedures to vet accounts that want to do a lot of paperless transactions against random accounts. I doubt you can go to the bank, set up your Acme Inc. account, and transfer money from 10,000 “customers” in one month, covert it into a cash withdrawal, and disappear - without setting off alarm bells before you even got started.
I don’t even know if merchants in Canada typically still take cheques - everything is debit card or credit card nowadays. The cost for a bounced cheque is so high, that even 20 years ago one of the local restaurants I went to would have a display behind the cash register “buy back your bad cheque” where anyone could read who had written an NSF cheque. Back then, IIRC, the fee was $15 or $25 or something. There was no real upside in taking cheques when everyone had a debit card for the ATM.
Of course, the numbers are still useful to Nigerians, apparently, so most banks have procedures in place to prevent transfers to foreign banks.