Cycling pants without reflective elements are perfectly fine if you only ride during daylight hours, race, or train indoors. Many road cyclists prefer this option because it keeps the design clean and simple, and reflective material doesn't do anything for you in bright sun anyway.
However, if you ever commute in the early morning, ride into the evening, or find yourself out in overcast or foggy conditions, pants with zero reflectivity become a real safety concern. Without those elements, you're relying entirely on bike lights and bright outer layers to be seen by drivers.
The key is being honest about your riding habits. If you're strictly a fair-weather, daytime rider, skip the reflectivity without guilt. But if there's even a chance you'll be pedaling in low light, consider at least minimal reflective accents—or commit to always pairing those pants with dedicated lights and a high-visibility jacket.