Because not everyone who is a good stealther can switch to another language to make bird calls and system window emotes that your party members can hear, and truly, this is a kludge with a shortcoming in the system, not a feature.
Because if you can already see them on the mini map, you already know where they are, and the only thing you end up hobbling by keeping them invisible on-screen is the ability to emote and RP.
On the IC rationale that, if you are allied with your party, and you're all traveling the treeline, so to speak, or in the shadows together, you know where each other is at. And if you split up, you're not trying to hide from your allies; you're trying to hide from your opponents.
And, if you're speaking, even if you're hiding, people are still going to hear you.
It's my new #1 on my wish list, now that companions will stealth with my PC.