I've worked for them for over 15 years and mostly take bonuses and I am still shopping for them so I can't say that you will have a problem. Now if you cancel a regular shop and pick it back up on a bonus you might have a problem.
There is no written Marketforce policy that prohibits only taking bonused shops. What you may be thinking of are posts that refer to shoppers being suddenly deactivated by Marketforce. In most cases no explanation is provided, so the information vaccum is filled by guesses from people who think they were too old, or took too many bonuses, or whatever other theory they can come up with. And perhaps they're right, and perhaps they're wrong, but we'll never know. I will say that I am called by schedulers at least five times a week with offers for bonused shops (usually exactly $5 less than what I previously did the shop for) and I have never been threatened with any kind of penalty for taking the bonused shops they are calling me to offer.