It's pretty cumbersome to have expressions work with dropdowns because currently you can only get the number value of a dropdown. Here's an example:
if (thisEffect("dropdown").value == 1) value = 13;
else if (thisEffect("dropdown").value == 2) value = 50;
else if (thisEffect("dropdown").value == 3) value = 100;
Now let's say you use the dropdown to control 12 properties. Then you make some mistake or you decide to change the values of this dropdown. Have fun.
SOLUTION
A solution would be to have a .textValue property of dropdowns. Here's how that would work with the example:
if (thisEffect("dropdown").textValue == "Style 1") value = 13;
else if (thisEffect("dropdown").textValue == "Style 2") value = 50
else if (thisEffect("dropdown").textValue == "Style 3") value = 100
Now the code is a LOT more readable and easier to work with.