The way I see it immediate actions and attacks of opportunity are a mechanical fudge to bring the sensable but narratively broken mechanics of breaking donw the combat in to rounds where one person makes all their actions then the next person. Otherwise combat becomes this odd thing where in six seconds, you you actually take 6 x the number of combattants, and most of the time people stand around scratching themselves and occasionally lashing out with their sword in frustration at the odd time/space dillation
I like PbP because we can actually sensibly pull back the narrative once the round's actions have been given, remapping the combat round as it is supposed to be - ie everyone is making their actions almost simultaneously, with a small delata delay between each person. So to me immediate action and attacks of opportunity make sense in that you are trying to do something yourself, but six seconds is a long time for a small amount of movement and doing one action. A lot of that six seconds is 'dead' time where you are parrying, moving in small sidesteps and forwards steps, (as opposed to the cummulative movement that is termed a five foot step) and actually awaiting the right moment for a strike. In that six seconds I can imagine that you have the timeand awareness to reflexively do something that is classed as an immediate action in reaction. Just like you always have time to make a hasty and most likely second swing with your weapon if the target you are focused on loses focus for that moment.
To me a readied action is not a reaction to anything, it is a deliberate gamble to pause in your own actions. Both mechanically and narratively.
So for example, in a hostage situation you ready an action - you will bean the guy with a spell the moment he looks like he's going to slit the hostages throat. So the second he twitches, boom. Except mecahnically (and understandably narratively) if he instead pushes the hostage forweards and runs away, you can't bean him. In a crowded moment you are deliberately pausing, that to me is a readied action.
Whereas say in a fight suddenly you see movement out of the corner of your eye, and reflexively you shout the words of a shielding spell (or suddenly srop down a pit and featherfall). You aren't walking around, not doing anything other than going 'oh god I hope I don't get hurt', it's just when a danger appears out of nowhere, or something unexpected happens, you have a mechanical actiont to use up when you want to say 'wouldn't my character react to that, since all this is supposed to be happening at the same time?'
Anyway, that's my thoughts on the difference between the two. If you want to drop them, you can, but I think you will find yourself fudging for them when you summarise a narrative - dropping down a pit causing the rection to come in the next round, or you'll stretch the dice rolls of a standard action to fit a reaction to something someone else did.
You could always compromise since PbP would allow this, and say that something that might have been an immediate action can be substituted for your standard action, because it interrupted it? However, I always feel 3.5 is a little punative with how much stuff you can do with a standard. So you might have noticed in my own game here I would allow some things that would be a standard action really to slip in to move-equivalent, and some move-equivalent to be doable as part of a basic move (jsut as readying a shield got folded in to that).