If it's like the SL-40, the intercom function is not voice activated. That's a big reason right there.
Hans
I'm not very familiar with the built in intercom function of the ICOM. The reason I'm going with a separate intercom is because I like the flexibility that it offers. For instance, it will auto mute any music source that is playing when a radio call comes it. Additionally, I will be able to connect my cell phone to it and make phone calls via my headset. Big plus, wife can listen to isolated music while sleeping while I'm listening to radio calls. Audio outputs from my EFIS will connect to it. Just to name a few.
Hope this helps
Actually, I am a proponent of going even FURTHER in that I believe it is now to the point where an "audio mixer" (Garmin, PS Engineering Audio Panel, or some standalone mixer plus intercom) of some sort is preferred. My reasoning is that with the multiple sources of sound in the modern machines we are building, we need a way to get it to our ears.