(Who can't easily hold down the left button so long),
Look into the windows "Click Lock" feature. It is a feature that will hold down the left mouse button for you at will (virtually).
If you have Windows 7 or 8:
Right click on your desktop. Click on "Personalize", "Change mouse pointers". Select the "Buttons" tab and at the bottom of the dialog box check "Turn on ClickLock". Click on Apply and Ok. This will work throughout all Windows programs.
An in-game version of clicklock might be a great (patch) addition for the developers as an alternative to, or addition to, WASD movement. It would work like this:
Add a key that, when pressed, will be toggle "automove". Automove would be the programmatic equivalent of telling the game that the left mouse button is being held down... until whenever automove is turned off.
PS, one more vote for WASD... without key movement, I cannot map game controls to a gamepad with "joytokey".