Never *had* to replace struts on a vehicle. That said, with very advanced mileage, (well over 300k,) the OEM struts on my prior vehicle would get a resonating bounce only under the specific conditions of frigid (sub-17°F) conditions and on a specific stretch of highway. This gradually increased with mileage but I was rarely on that stretch of highway and under those frigid temps.
I've known some folks to replace their struts + springs when the reality was, they only needed an alignment. Struts are what keep the springs from going bouncy-bouncy; they dampen natural spring movement and potentially prevent over-extention - which can unseat and/or break them. (note: struts would have to physically break to allow over-extension) The springs control ride height and ride quality.
If you're bouncing around like you're in a Snoop Dog music video, that is struts. Struts can be tested when removed from the assembly; there's a good YouTube video how-to. If they're oil-based struts and are visibly leaking then they need to be replaced.
If it feels like you're riding on the ground, slamming on normal bumps, or bottoming-out, that is likely spring related.
The ST's stock strut on particular road surfaces, has been known to resonate and be particularly bouncy. So you'll have to know what "normal" is before you go assuming that they're in need of replacement.