Read the terms and conditions carefully for 888. The recent change implies that you can't wager bonus funds until you have used all cash in your balance. Withdrawing any cash from your balance forfeits the bonus. So it would appear they want you to go broke then use the bonus to try and clear the bonus!
I have read some reports online of people registering for future tournaments to get the cash balance down to zero, playing the bonus money, then unregistering from the tournaments to get the cash back. I suspect this would be viewed as abusing the system and could result in your account being frozen.