<p>OP: I have known people who have double majored, combining engineering with math, economics, or even other engineering disciplines. As you acknowledged in your post, it has no real advantage in industry or academia and is not in my experience worth the investment of time and money.</p>
<p>But.</p>
<p>It is your life, your time, and your money. If this is what you want to do, and you understand the requirements, costs, and impacts, then I see no reason why you should not. As others have noted, you can certainly study math without taking a full math najor, but depending on the school you may find greater access if you declare the double major - some of the most popular classes are usually reserved for those in the major. I would say that with the right selections, a BS in engineering + MS in mathematics may well be as long or even shorter than the double major, and more advantageous, but you need to investigate to be sure.</p>
<p>I do think that turbo93 has offered some sage advice - be aware that the length of time to complete the double major is highly variable dependent on the rules of the school / departments as well as the specialties chosen in each major.</p>