No one is arguing your points...the disagreeable point that was made was that a "true professional" would not use an iMac. There are numerous professionals that I know personally who DO use iMacs...very high-end iMacs, but iMacs nonetheless. And are quite successful with them. And they have massive raid storage attached so internal storage is not an issue.
That said, there are applications where a Mac Pro would be more useful, where you may want more on-board storage, even higher-end graphics, more memory. One has to choose the appropriate tool for the application. But a blanket statement that a true professional cannot use an iMac is just plain a false statement.