There are many situations in which this combined question mark/exclamation mark would be very useful. Scriptwriters, for example, might make constant use of if for people's outraged querying of things...ie where they are presenting very loud questions.
Technically, however, rhetorical questions which are presented emotionally or vehemently need only an exclamation mark at the end and not a question mark as well. For example: "How can people do such things!"
"Sometimes, especially in popular writing, the two are combined for emphasis." ('The two' refers to joint question marks/exclamation marks...?! or !?)
The above quote is from Fowler's 'Modern English Usage' - one of the ‘bibles' of usage - so it would clearly be wrong to say one should never use these marks together. However, I would still advise restraint in using any such piece of punctuation. Note the words 'sometimes' and 'popular' in the above quote. They suggest that - even if you are writing for 'The Sun' - such a thing should be a rarity!
Personally, I would never use it. Even the exclamation mark alone is - according to The Times style-guide - "almost always unnecessary".