Playing with accounts which are not yours is like cheating on your taxes:

It seems ok until you get caught.

Sure, you can find a lot of people who say "I use a friend's account and I have had no problems". But, I would encourage you to lurk in the official customer service forums for a few days and you will see that

1) Blizzard is pretty clear on the TOS in this case

2) Blizzard rarely unbans accounts. The few instances I have seen have been true mistakes (characters with similar names and such).

So, the question to ask is: is it worth the risk IF you get caught?