Must have been a while since your last MT update
<ramblings of a VT developer follows, feel free to disregard>
One consideration of making maps in a VT (like MT) vs outside is the limits of map size. Creating a full size image for the background can cost a lot of memory, especially at high dpi. Which also means more data that has to be transmitted to each player (particularly in VTs that don't send the compressed version of the map, such as BRPG iirc, please correct me if this is no longer the case).
Whereas if your scene has a lot of duplicate images you can create the same scene directly in the VT with very little memory by using the same image for each instance, regardless of rotation and scale (this is what MT does). So you could conceivably have a forest of trees, with maybe 3-4 derivatives, that goes on forever (well, within reason), and takes no more memory than those 3-4 images.
One advantage of making the map outside of a VT is the ability to use the resulting map across (nearly) any VT. That is, a jpg is a jpg is a jpg. Whereas if you make a map in a VT using that VT's tools, it will only work in that VT.
One advantage of making maps inside any VT is the "rich" features you can build into the map. Such as the ability to move chairs and open doors (or burn stuff down, as the pyros in the house are wont to do ), or have vehicle combat that spans several 10s of miles (or more!).
Over the years I've seen both style executed extremely well. For example, I recently opened a user's campaign to help debug some speed issues, and was in awe as I zoomed out and looked at the extent of their cave system. It was huge, and immaculate. And all built directly in MT using drawing tools with textures, and stamps. On the other extreme there are users that build amazing jpg map with tools like dundjinni and photoshop, that look like I could reach ouch and touch them.
Different GMs are comfortable with different styles, I think both methods are valid and that it's valuable to support both (and everywhere in between ).