Bonuses are great. Some books give you Bonus Plays (free money). You simply take a dog at the book that is giving you the Bonus Play, and arb it with Matchbook until you get your money in Matchbook. It's free money. Your bankroll is increased overall.
If you take the standard 20% bonus with a 2X rollover (which would actually be a decent bonus), you would have to wager 200% of your bankroll and pay 10% of that in vig on every wager. That's 20%, equal to the bonus. However, if you position yourself to make wagers that are +EV compared to Matchbook, you don't spend any of the juice and the rollover will be accomplished without problems.
You don't pay vig when you bet +145 and the closing number at Matchbook is +133/-134. You don't pay vig when you bet -120 and the closing number is +134/-135. The other side is simply paying 2X the vig. Betting these numbers at books that give you bonuses is an insanely profitable scenario. You're betting into numbers that not only should increase your bankroll but also hit that rollover and gaining the 20% bonus.