The funny thing is, if you ignore charging, it works exactly as (move-action + partial action), which can be exchanged for (full-round action + 5-ft step). I do believe that charging is the only bastard child amongst all the possible actions
Untrue twice over. You're forgetting sequence of actions and partial runs.
The Ready action lets you do MEA,PA, sure -- but not PA, then MEA. So actually using the printed rules for Ready, the turn is not really very equivalent to PA+MEA unless you basically ignore the printed rules and intent for Ready.
Also, several people are forgetting the partial run, used to run around corners. Then there's the weird question of whether moving (MEA) then running (PA) counts as "if all you do is move..." for the purposes of the AoO for disengaging.
To throw a little more sand into the logic of the argument that "Ready makes it so that a SA might as well be PA+MEA"
(1) Minor Logical Jusitification for SA != PA+MEA
Ready, when properly used, has a good chance of costing the character initiative. I.e., the MEA,PA sequence taken by a non-abusive Ready action actually takes
longer than a character's normal turn. And that is why more can be accomplished, and why Ready does not make a standard action equal to an MEA plus any old partial.
Note that Ready will not always cost you initiative, especially in contrived 1-on-1 example situations. In real play however, using Ready legitimately will often cost you initiative and slow you down, unless you allow bogus triggers.
I'm not saying that just going whole-hog on letting SA=MEA+PA will destroy the game, although I do think you need to work out what running and charging really mean.
IMC I like my players to just explain what they want their characters to do, and then I translate it into game terms, and show them how much of it they get accomplished in their turn. The
major culprit of this style of play however is the 5' step, which has such incredible tactical importance that players generally have to know the rules for it.