Thanks for the good explanation on this.
Now I totally agree with you we need to calculate the taxes on pro-forma button also because as per the following scenario.
Company 's accountant has created the invoice and set this as a pro-forma but forget to "Compute Taxes" then he print the invoice, sent to customer and waiting for this approval but sent invoice doesn't contains the taxes in this scenario.
After the approval accountant going to validate the invoice, now he will see the taxes on invoice so It will becoming a strange because customer doesn't know about the taxes. That's this will making a misconception between company and customer.
One more point we have to remove the Bharat's fix because It's useless, If we have properly fix this. So you are correct in your comment#1 this fixes is strange.
Now I am going to work on this and will fix asap.
Anyways I appreciate your passions on this issue.