I use a keggle for my HLT and a megapot for the main boil. I find that the megapot boils the same amount of liquid much faster than the keggle with the same burner. However, I was able for find a used keggle ready-to-go for $50, while the pot set me back 5 times that much. Personally, I like the setup, as I am more concerned about my wort boil times than my HLT water heating time.
There's no harm in getting a keggle to start with. If you decide you don't like using it as a boil kettle, just use it for your HLT instead!
Greg