I use synthetic for warm weather insulation and have used it for colder weather as well (as far down as 21* F). That being said, the hydrophobic down coming out now (the only cottage manufacturer that I'm aware of using it is Underground Quilts
) may make two of the major synthetic advantages moot: their resistance to moisture and their hypoallergenicy (my spell check insists that that's not a word, but...). However, synthetic's price advantage is actually increased
by the hydrophobic treatment, so that may be a wash for you.
For me, the main reason I went with synthetic is fairly pedestrian: price. A poncho liner is ~$30 and good to ~40* for me, at a weight penalty of ~0.75 lbs over a down quilt of the same rating. Plus, it won't collapse on me if it gets damp from ambient humidity (very likely down here in FL above 50*, which is where I'll be using it most of the time).
The modular insulation system I'm currently working on uses synthetics for its warm-weather portion, specifically because the insulation is a smaller percentage of the total weight of the quilts at those temperatures. This lessens the total weight penalty, since the weight advantage of down at those temperatures is less. It also increases reliability.
The bulk penalty for me is less of an issue due to my pack; I use a modular DIY pack inspired by the Molly Mac Pack design. So, I can vary up and down on volume constraints fairly easily. For someone with a fixed pack volume, it may be worth going with the smallest packable items possible, meaning down.
Anyway, those're my reasons. YMMV, and all that.