Is it okay to still drive a car when the struts need to be replaced?

The primary purpose of struts and shocks is to "dampen" the unsprung weight of the wheel assembly to prevent bounce. In other Words, shocks and struts stop the wheels from bouncing when you hit a bump.

Without shocks or struts, a wheel would bounce like a Basketball when you hit a bump, and the wheel wouldn't be in contact with the road very much of the time. That makes the vehicle unstable and difficult to control especially when you're going much over 15 MPH.

As opposed to shocks, struts also include suspension springs and bushings, so it depends on which part of a strut is going bad. If the springs are getting weak and the vehicle is "sagging", that's probably not too terribly unsafe, just an annoyance. If the bushings are shot, don't drive it.

In my opinion, it's not a good idea to drive without good struts, but that has to be your decision.