Of course I don't do either degree, but from what I heard at Open Days contradicts what you've just said. From memory (going back a couple of years), it's become a popular combination because traditionally engineers didn't have as larger a role in the managing and finance matters of the firm, or various projects. Of course this wouldn't give you a massive job advantage, but I guess the skills would be useful.
Also, I think there's a reason why in any Monash Eng degree (incl Aerospace Eng) that there is a compulsory Comm unit in first year (I think?). Probably has some correlation with the above.
Also, you're assuming there aren't students who actually have an interest in both Comm and Eng and would just like to get into either a bit more than in a single degree (such as the OP perhaps).
Nah, engineers have always had a role in the management, I'd just argue commerce degrees aren't the way towards that.
Generally, outline of engineering professional's life would be something vaguely like this:
Start off as a grad engineer: ~3 years
Work towards becoming a lead engineer: ~6 years
Lead/Senior Engineer: Indefinite, from here you try and move into management if you want to make a lot of money.
The only step where a commerce degree could possible assist you is at the Lead/Senior Engineer part. I'd argue, however, that management skills are something you gain after a while with experience and stuff rather than as part of the degree.
However, if people do like commerce and do like engineering, sure do a double. But eventually you will need to make a choice between the two. I simply feel it's better to make that choice before you start your degree so you can devote 100% of your efforts to your field and become really good at what you do.
@B^3, whether you lighten the load or not, you're still getting the same out of your engineering degree. After you get your first engineering job, your grades become instantly irrelevant. If you're a good engineer, you'll shine from there on, if you're not, you'll fail in the profession. So, if you were worked harder during your degree, it's better for your prospects.