What I see time and time again on SF is people think that there are style rules that are set in stone- there aren't.
As for the quandary with the penny loafer. You can have quite a bit of fun with it, if used properly.
No, they are not a good idea with a dark (almost black) and formal suits. However, they frequently look excellent on summer weight, light colored and cotton/linen suits. For many summer functions, they very easily work well with these suits. On the other hand, they rarely- if ever- are appropriate for the business setting.
Furthermore, it seems like the trends is to downplay the formality of dress (knit ties being popular, as well as the button-down collars even in the business world). Maybe loafers are along those same lines. Sometimes you have to trust your own judgement on the matter.
Lastly, the penny loafer is certainly not a fashion forward shoe. Many on this thread have even deemed it an "old man shoe". I tend to disagree, it's just a shoe that has stood the test of time- along with the cap-toe and wing-tip oxford. The fashion forward man might opt for a driver, which I agreed look better on a younger man. However, drivers are significantly more casual. They seem a bit more difficult to put together in a suit outfit. But again, the rules tend to bend with summer functions.
I have a pair of penny loafers in my regular rotation. I wear them almost exclusively with jeans, and pair them with just about anything on the top. I think penny loafers can be a fashion forward look - assuming they're of a more modern, "European" styling than, say, Weejuns. I'm 26.
What is the fit like for you on these? I ordered the same/similar pair on Amazon recently, in a 9.5. (I normally wear 9.5-10 in US sizes). They are too big, but I can't decide if it's worth trying a half size down.