I have thought about this too. Say, for example, I agree to roll my money over 5X to earn the bonus. I deposit $1,000 with a sportsbook and get a 20% sign up bonus. After I've wagered $6,000 and earned the bonus, I get nothing. There is tremendous incentive for me to now pull my money out and negotiate another sign up bonus with another book.
Having said that, I am still a fan of the cash sign up bonus. I always increase my wagers for the season by the percentage of the sign up bonuses I expect to earn. For example, if my bankroll is $10,000 and I get $2,000 in bonus money, my standard wager increases from $300 to $360 per play.
Guardian Guaranty pays 10% interest on all account balances, unless the money is in their 105 club. I love that. This little gimmick has caused me to leave money with them after football season is over...money that I would have otherwise withdrawn. I have yet to make it through a season of MLB or College Hoops without betting, even though football is the only sport where I think I have a real chance at winning.
Later,
osu2winbig12