Helmets without rotational protection systems still meet all required safety certifications—they're not unsafe. They rely on standard EPS foam, which does an excellent job absorbing direct impacts. If you're on a tight budget or just need a helmet for casual neighborhood rides, these helmets will still protect you in a crash.
The trade-off is that you miss the extra layer of protection against rotational forces, which occur in most real-world cycling crashes when your head hits the ground at an angle. Without a system like MIPS, there's nothing specifically designed to reduce that spinning motion transmitted to your brain.
That said, helmets without rotational systems often cost less and can offer slightly better ventilation since there's no internal liner to impede airflow. Some aero helmets also skip these systems for aerodynamic reasons. If budget allows, rotational protection is worth having—but don't skip riding just because you can't afford a MIPS-equipped helmet. Any certified helmet is far better than no helmet at all.