The power panel on the starter box doesn't drain your batteries...the motors take way more current than the power panel. The power panel still has a voltage of 12-14.4v at the end...that's 12-14.4v to the glow plug BUT it has very very low current...much much lower than 1.2v ignitors which tend to give as much current as the battery can produce.
Imagine the difference from a small AA battery and the D battery some guys use.The difference in the current could be up to 3 times more.
This can ruin the glow plug any day.
Plus the power panel has a regulator to adjust it according to gel cell and stick packs.
It also has a light that enables you to monitor the "life" of the glow plug...no light no glow!!!!