Are you accustomed to pushing the exposures on your RAW files? Usually I do because the E-TTL II system is very finicky. Imagine my surprise when I tried to correct an ISO400 image by one stop and noticed that the shadows were falling apart! The shadows appear much darker/contrasty before correction and remain that way even after correction.

That said, it does work for highlights. For a scene that's overexposed by 1-2 stops, you can pull back to retain highlight detail, while the RAW file without HTP loses it no matter how much you pull back. That means the wedding dress or backlit faces with highlights on the cheeks while outdoors should have much more recoverable detail with HTP on.

I know that Canon markets Highlight Tone Priority almost specifically toward wedding shooters, but you should use it only when you have to worry about highlights and leave it off for other situations.