Each method confers a slightly different advantage, though the actual LHTR/OM rule of comepleting each combat before going to the next actually confers MORE of an advantage than rolling the combat all in one mass (as you could if you made multiple combat entries using the in-site dicey).
I agree with Dan. There is no real advantage conveyed when rolling every combat at once (afterall, combat is simultaneous under the rules…)
I think it should be left up to the players in a given game if they want to to one combat at a time (such as occurs with DAAK, Frood, or can be done with the in-site), or if they want to allow all of the combat to be entered and the coding for various battles keyed in to a single post with the in-site dicey.
If players cannot agree, then the default position should be one battle at a time.