Well first off I see you are using = with the /set command. You don't use = with /set. Also you can't do math using /set. You can do two things really.

Use /var
which fits the syntax you are using
Code:
test.tertiary  {
  var %value.test = 5
  ;Set variable (value.test) to the number five.
  echo -a %value.test
  ;Display variable (value.test).
  ;Output:  '= 5'
  ;Desired Output:  '5'
  var %value.test = %value.test + 1
  ;Increase the value of variable (value.test) by one.
  ;I know there is a command for this, but for testing purposes the equation is more important than efficiency.
  echo -a %value.test
  ;Display variable (value.test).
  ;Output: '= %number + 1'
  ;Desired Output:  '6'
}


Or /set
which requires the use of $calc for the second part
Code:
test.tertiary  {
  set %value.test 5
  ;Set variable (value.test) to the number five.
  echo -a %value.test
  ;Display variable (value.test).
  ;Output:  '= 5'
  ;Desired Output:  '5'
  set %value.test $calc( %value.test + 1 )
  ;Increase the value of variable (value.test) by one.
  ;I know there is a command for this, but for testing purposes the equation is more important than efficiency.
  echo -a %value.test
  ;Display variable (value.test).
  ;Output: '= %number + 1'
  ;Desired Output:  '6'
}


I personally prefer /set but many people seem to loathe it. Oh and if anyone says it's an documented feature, it isn't any more. No clue when it made it in the help file, but that was an argument against it for a long time.

So really it's up to you which you'd like to use.

Also about variables being numeric or text, I don't believe mirc variables have a type unlike other languages. If you end up passing text to the /inc command for instance it just does nothing. So it's just up the user to keep track of which variables are what type or add checks if needed.

http://scripting.pball.win
My personal site with some scripts I've released.