A twinlinked heavy bolters with reroll to wound kills on average 0,79 MEQ's each turn. Cover doesn't matter
A twinlinked multi melta kills on average 0.74 MEQ's, if the target is in cover this is reduced by half.
Ofcourse I know that you'll most likely will be shooting at vehicles with the MM, but the heavy bolter immolator has its uses. Also it's yet another vehicle on the table so either it will be ignored or it will draw fire away from the other vehicles.
Also a MM immolator costs extra points AND it will be much higher on the target priority list.