You just listed two massive positives of doing comm/engineering over a straight commerce. What are the negatives? And no, "spending more time at uni" is not a negative. Neither is money, especially if OP is interested in banking. Also you're arguing that you can have the same amount of exposure to professional development in a 3 year degree, compared to a 5.5 year degree? That's definitely not correct. You can do so much more in that extra 2.5 years.
I think it should also be noted that (at least at usyd) the engineering work experience does not need to be done in the penultimate year, it can be done between years 3 and 4. This leaves the penultimate year free for anything commerce related. Alternatively, as people have suggested you can do some finance related internships in between 3rd and 4th year. I have also been to some of UNIT's seminars (including one by engineering graduates now working at Macq) and when I asked a recruiter there about the dilemma engineering students had, she said that we won't get ruled out just because we're not in our penultimate year and still suggested we apply.
Also OP, if you don't like engineering then definitely drop it. It's not a type of course where you will do well if you don't like it. But if you like it/find it ok or bearable, then I would say stick with it.