Seanothon,
You went "straight into DM'ing", the moment you got certified as a DM. You are now a Dive Professional, whether you like it or not. Again, your instructor should have made this VERY CLEAR through your entire course. Again, this is about CYA, especially in the presence of other divers that may or MAY NOT be under your supervision, or in your group, on the same boat, etc. Having your own insurance helps to protect you not only from yourself, but from "the other guy". We live in a very litigious society. God forbid anything should ever happen, but things do, and so do people. An unofficial "guideline" that is often put out there for DM's, is to never dive with anyone else that is not a DM or higher, when diving for fun. That's your choice. There are a thousand different potential hypotheticals and/or scenarios. Comprende ? I am not advising you what to do, just sayin'. When you get your drivers license for the first time, the first thing you do, is get insurance, whether or not you're driving or have a car. Make sense ? And again, I'm not advising you what to do, it's your choice. I'm just sayin'