Because coordinating use of a serial channel is easier than using separate channels?
Only in special case apps. Otherwise more trouble than it's worth.
As for Strings... you have been shown again and again about those and Arduino.
Yes, there are BAD practices that can be gotten away with. And then comes the help needed posts where as usual part of the fix is an OP UN-learning stupid/wasteful/bad habits before they can write their non-trivial projects.
Damage control? Yeah, to try and cut the HOURS WASTED over stupid/wasteful/bad practices.