Don't negate the possibility they may not know what they are doing.
It depends on the synth dynamic and the intended application.
You can control the volume of the sound over time as the timbre changes. For example, a sound that starts out as a high frequency and then goes down to a low frequency may have a different amplitude throughout this change. Compression will bring these high volume parts down to the low volume parts, ultimately allowing you to have the sound at a steady volume throughout the mix. This maintains the focus of the track as your attention isn't suddenly drawn to a quiet section (or vice versa) in a bassline or synth part.
When you understand what a compressor does, you can apply this to any situation. Whether it be a drum, bass guitar, a supersaw or your uncle's foley collection. You'll be able to assess the situation and apply your own discretion.