That's not quite correct - you could - or at least I could and I'm sure a lot of other explorers could - tell right away if that planet was a Water World, or that a planet that looked like an ELW was in fact an HMC and not an ELW, just by selecting it on the System Map and listening to the soundscape. You can still do that now after building up a system map.
So really, the FSS spectrum thingy is in place of listening to the sounds in the system map. The former is visual, the latter was aural.
I'm getting better at using the FSS, but I have hearing loss, and cannot distinguish signals sometimes, especially when they are close together. Aural cues are challenging for a UI to mesh with a densely packed visual display, like the spectrum display.