Imagine these two scenarios, 1 where a silent mode icon exists, and one where it does not.
_1) No silent mode icon, just the red dot on the switch_
10pm - Go to bed and set my phone to silent
6am - wake up and go to work, forget to turn phone off silent
9am - check my email on the iphone (don't look at side of phone, who looks at the side of their phone? but i sure look at the front all the time, i have to in order to use it)
10am - miss a call since phone is on silent
11am - miss another call
2pm - miss another call
3pm - check email again, see missed calls message, oh no, i left my phone on silent by accident and missed 3 important calls!
_2) Silent mode icon at top of iPhone screen (status bar, visible when phone is both locked and unlocked)_
10pm - Go to bed and set my phone to silent
6am - wake up and go to work, forget to turn phone off silent
9am - check my email on the iphone, see silent mode icon, oh no, forgot to turn off silent mode, better turn it off now just in case i get any calls.
10am - don't miss a call since phone is not on silent
11am - don't miss another call
2pm - don't miss another call
3pm - check email again, no missed calls, good thing I saw that icon reminding me that my iPhone was in silent mode!