Get licensed. You will then have more opportunities to buy or list properties. When a great deal comes along, buy it. When a FMV deal comes along list it.
The biggest reason "not to" would be costs. Cost of training, cost of "hanging" your license at a brokerage, then yearly costs of E&O, continuing ed, and licensing fees. Generally the hanging is the most expensive part, but you can find low-cost brokerages if you search. If not careful, though, it could eat up that $300/mo positive cash flow.
Other than that, I can't think of a reason to not do so. Having it might also open additional opportunities to you, as @John Thedford mentions. That could certainly offset all the associated costs, and more.