Making that side exit a one-way thing would, on the one hand, hinder ninjaneers and pyros and spies and other such unsavory characters that would make me feel unsafe coming out of spawn. However, I didn't feel particularly unsafe at any time during the last playtest, despite this side entrance, and on the other hand, this would make the run to the resupply cabinet and health, precious health, that much longer.
When it gets down to it, I'm not sure that a one-way door there would quite be worth it.