The drawback is only that currently all of the highlights for the entire synth will be shown in one extremely tall column on the right side of your synth, so if you add multiple highlights per photo to a lot of photos, it may be difficult to find the highlight that you're looking for. I guess since they are called highlights, rather than tags, they are meant to be used sparingly.
It would be a bit nicer if when you were close to a photo, only the highlights for that photo would display, or they would simply appear when you hover your mouse above them, but the Photosynth viewer isn't quite there yet. Give it some time, though and I think we'll see the highlights grow into more of what you're after.
In the mean time, give highlights a shot and let the rest of us know what you think or what you'd like to see added or changed.
Cheers, mate, and congrats on getting married. =]