again, i'm not speaking on behalf of the rotman school, i'm just airing out what i see as a student here.
i can't speak for investment banking, as i'm not going down that path. but for consulting, i would say that yes, it's much more difficult if you don't do a summer internship. the question they'll ask is why. is it because you couldn't get one and gave up? is it because you went and did something else, like a start-up? it all depends on what you're doing over the summer, and if it is impactful and career-progressing. even if you don't get into a external consulting firm over the summer, just doing a strategy internship somewhere else (think corporate strategy for a company or something along those lines), it would help you tremendously for full-time recruitment especially if what you did is similar to the experience they're looking for.
also remember that rotman is increasing the size of the class. more students every year, but about the same number of placements (at least for consulting). you have to be increasingly competitive, and i think this might be why internationals are finding it difficult to get a placement. i mean, it's not just the culture and language, but the networking too. an international student arriving in august before school starts will have no network. whereas someone like myself who has been in toronto a number of years may already have an established network to build on (prior to even considering doing an MBA, i already knew at least one person in most of the major consulting firms).
for one-year MBAs, i'm guessing that you'd have to rely on your pre-MBA work experience. i think it's also more difficult to switch careers because you don't have that internship. not only for full-time recruiters (since they don't see your experience), but for you too, since you don't get to "try out" that career path. i'd say if you're switching careers, do a two-year MBA. my opinion though.