I have poured various syrups into actively fermenting wort (usually as krausen starts to slow) and have never had an infection from it (and several of these beers have been long-aged bottles). I believe it is a good way to keep the yeast happy and maximize dryness and yeast health for very high gravity beers - the yeast goes through its growth phase in healthy wort and then eats the simple sugars later, as it is still active, but most of the maltose has been consumed. That said, when I am brewing 12 gallons of tripel, I usually toss the syrup into the boil ~10 min before flameout.
For me, adding syrup into the primary allows me to split a 12 gal batch nicely - half can be a lower-gravity American, British, or German style, and half can be a higher-gravity Belgian ale. Splitting a ~1.048 Kölsch and giving the other half a pound or two of D2 and WY3787 can lead to a more diverse kegerator. If I noticed infection coming from the syrup, I probably wouldn't do it again (or I would boil the syrup with an equal amount of water before adding it), but it has been a nice option for me so far.