We probably won't add this. The performance overhead of running a trigger is fairly small, but variables are used and changed so much in projects I'm sure a feature like this would drag down performance. And the workaround is simple: either use a static local variable, e.g.
[Static local number OldValue = 0]
+ Myvalue != OldValue
-> Set OldValue to MyValue
-> Do something else
Or instead of setting the variable directly, call a function, which does additional work as well as setting the variable.