Actually that is valid use, "set /a" doesn't require variable names to be expanded(except dynamic variables like %random%):
From "set /?"
.... Any non-numeric strings in the
expression are treated as environment variable names whose values are
converted to numbers before using them. If an environment variable name
is specified but is not defined in the current environment, then a value
of zero is used. This allows you to do arithmetic with environment
variable values without having to type all those % signs to get their
Furthermore using a standard environment variable will break the script because of variable expansion issues.
Valid, equivalent and functional it the situation:
set /a count += 1
set /a count=count + 1
set /a count=!count! + 1