On Planned Encounters vs. Random Encounters

So is it better to have planned random encounters, or random random encounters.

I'm running an old school Mega-dungeon for several groups (Numenhalla) and have been wrestling with this question often.

If you look at old school encounter tables in modules, there are many entries that consist of things like "2d4 zombies" and "2d6 Gnolls + 2d4 Hyenadons". Then upon the encounter you roll for the 'reaction' and role-play from there.

But what is the harm in having more interesting encounters preplanned. How many random encounters will the average party really encounter? And don't they basically fall into several categories (Things we fight, things we avoid fighting? Things we talk to?) And if the encounter is a more positive one, isn't it better to have say a name for the leader of the Gnolls, or the Maneaters?

I've done my table in the old school style and during play I've found it to be lacking. I have a general table for each level, with specific blanks for each encounter area. After a week or two of play I created a 'special' encounter list.

Here are the things I've noticed

• Have the table split makes for a bit of page flipping and no centralized monsters information location.
• I consistently make too many entries on the charts, meaning everything feels random instead of themed.
• There just aren't that many random encounters that happen in a single night. I generally roll between 12 and 20 times in a 4 hour session, meaning, on average there are just 3 random encounters.
• Being that this isn't (strictly) a wilderness, things that made sense in a wilderness sandbox (such as many monster 'animal' encounters) do not make as much sense in a dungeon - they feel less organic.
• I'm not saying I have a specific plan for each encounter before it happens, but I often think a little more assistance with increasing the relevance of the encounter to the party, and ways for them to hook into that could be helpful. (Each player asks, "Why should I talk to dudemar?" They should have an answer to that question).

As an aside, in a megadungeon, random encounters are less a control for mages and the fifteen minute adventuring day then they are in sandboxes. One way doors, trick stairways, teleporters and things that trap the party seem to take that role. Considering that when they are covering new ground it takes 1 turn to move the slowest movement of the party, every time they move 180' they trigger a check - but when they are moving back over covered ground, they are moving quickly, and not checking for traps and such. Checks being based on time do little to address the fifteen-minute adventuring day that way.

So this bears examining - what is the purpose of the random encounter in a Mega-dungeon?

In spite of this, my party did managed to get trapped between a Dragon and a Group of Anthrophages in a treasure room on level 1. A group of anthrophages led by someone they managed narrowly knock out last week - so by no means has what I've been using so far been a failure.

Let me know your thoughts on the purpose of the encounter in the dungeon/megadungeon - and on what ways you might structure your table uniquely.  I'll talk more on my redesign to make the tables more useful soon.