The only difference I know is when you need to cross-flash.
Such as when you want to flash RM-356 firmware into a RM-428 phone.
If Phoenix detects the phone it will block you from flashing firmware into a non-matching device.
Using dead mode causes Phoenix to skip this check.
This said, I don't know why every CFW guide suggests dead mode.
I've been using regular mode forever and never had a problem.
Dead usb flash is design for flashing when ur phone is dead (infinitely reboot, or sometime stuck in "Product in flash mode" like that)
To avoid some member skip asking why they can't flash their phone, so use dead usb flashing is better.