Proportions and percentages, as I understand them, show the same information. So why report one over the other? Or perhaps what I'm really asking is "Under which circumstances would one be preferred over another?"
I believe this depends on your audience. Personally, I dislike proportions in the form $p=0.00001$, when they are smaller than $1\%$ -- since I have to count the zeros to understand the number. However, the same is true when the upper number is expressed as a percentages $p=0.001\%$. It's still hard to understand the quantity. In these cases I prefer $p=10ppm$.
However, if I am talking to non-statisticians (e.g. students) I use neither percentages nor (standard) proportions in the form of a single number. Instead, I try to use proportions in the form $X$ in $Y$. For example, one person out of thousand instead of $p=0.1\%$, or $10$ pieces out of 1 million. It seams as if this is more intuitive than a percentage, especially if you talk to people who do not feel comfortable handling numbers.