Should supports be used as the primary weight of your attack?
You've done a good job of detailing the main advantages of using a support order, but you've left out a key concern when using them: enemy raid orders can remove support orders, so your supporting regions would need to be buffered against enemy-controller regions of the board. You also rightly introduce the fact that the march orders are the only way to get your troops to move around the board.

Another somewhat related aspect is the limitations on the size of your armies based on the supply track. If you have one or two large armies supporting a number of single unit forward positions, you can make much better use of the larger armies if they are supporting forward positions.

One other thing to consider is that there are a number of hero abilities which negate or change the effects of supporting troops, which don't affect the troops in the region doing the attacking/defending.