Local variables in preparation - why not?

Local variables in preparation - why not?

  
I couldn't find the answer anywhere on the site, so I'm asking here: why can't I have local variables in a Preperation? I often have temporary values that are only applicable during the preparation, and I don't like to polute my screen with locals just for that purpose. But that seems the only possibility. What's the rationale behind this?
Can you give an example of local variables only used during the preparation?

I'm guessing you can refactor the code easily not to have to use variables only during preparation, but honestly I cannot find an example where you don't need to use them in the screen/logic itself
For example a screen that is used for both edit and new of some entity. In the logic, in case of edit you fetch the current record, in case of new you initialize a new record. Now furtheron in the logic, you need to test some values for further initialization or screen logic, but in case of edit you need data from the current record, and in case of new you need data from some other record. A local variable that could be assigned in both branches can take care of that.

Another example would be when you need to calculate some value, and do a series of checks based on that value. A local variable could be assigned the result of the calculation and then be used several times. If this calculation is only needed for the preparation, and not further in the screen logic, a local variable is desirable over a screen-global one.

Whether you can or cannot honestly think of a good example doesn't mean there isn't a good example ;).