DSS keys allow you to pick up the line. Just like a multi-call, except you have to tap the button. It supports Ring or No-Ring, but no other ring options. If that was all it did, I'd agree with you. However, it doesn't just do that plus 'in use.'
The light can indicate idle (off), busy (on), ringing (blink), forward or DND (fast blink)
When idle, it acts as a speed dial key. This alone makes it more useful than a multicall on sets with fewer keys like the 5320.
If you're on a call, it acts as a 1-touch transfer key. Using DSS makes it a supervised transfer, or you can use a 'Secretarial Key' which is identical but is a 1-touch blind transfer.
If the owner of the line she has a DSS key for is on a call, the line is engaged and the secretary can't see any more incoming calls. Not one secretary/EA has ever told us she was happy with that, and they always require extra line keys to manage the incoming calls (which voids the idea of DSS allowing "fewer keys").
Plus, if they want to forward the call to the boss, they generally want to send it to the Boss private line anyway.
A multiline key doesn't fit the bill either though - the biggest drawback being if the Boss and the EA are on different controllers they can't share a MLK, plus she can't see if the Boss line is in use using a MLK.
Still, if the EAs could only have one, they would always choose the MLK over the DSS.