While some of the limits placed on tag blocks and tag data don't have much meaning (other than maybe exceeding the maximum value of what indexes it, say a 16 bit integer), there IS a meaning for the limits applied to object placement blocks. The game only allocates a fixed set of memory for all game objects. Every object consumes this memory when instanced. This includes projectiles, placeholders and garbage (ie flood).

Now, imagine an intense scene with 1 player, then a good sized marine squad of say 9. Now, the player is driving a tank, who has 2 marines with him, with 2 more in a warthog then 4 more in another tank.

The enemy includes 2 shades with 2 grunts manning them, 2 banshees with 2 elites, 2 wraiths with 2 elites again, then maybe 3 sets of 1 elite and 4 grunts. Then you have maybe 5 jackels running around.

Now, we're going to assume that every biped in this scene has 1 weapon and 2 of each grenade type. Except for the player, he has 2 weapons and 4 of each grenade type. Then you have to include the "weapons" which are attached to the vehicles (ie the shade's plasma cannon)

Then we're going to assume there are 15 different equipment pickups nested in the scene (heath, rocket ammo, active cammo, etc). The maybe 5 more weapons just scattered and not being used by anyone.

While the grenades don't add to the object memory until actually thrown (just like projectiles from a weapon IIRC), they are still going to be counted when actual battle commences so they're a huge factor (especially if it was Halo 2 and we had Catch on ;p)

We end up totalling in about 15 equipment, 55 weapon, 36 biped, and 9 vehicle objects. 115 base objects. Now lets assume on average that each biped is shooting off 10 active projectiles (grunts with needlers would probably raise this average dramatically). Thats 475 objects total thus far for this battle scene, not considering any grenades or the actual scenery.

This scene takes up about 20% of the object memory. Now, you have to factor in what the scenario has placed. This includes (sound) scenery, devices, etc. In CMT's detailed case, you've probably got the rest of your 80% of the level right there. 80 vehicles BY THEMSELVES (no other spawned objects, not even a player) take up about 7% of the memory. CMT's highest vehicle count I think masters said was 115. That alone is bumped up to 9%.

These limits aren't meant to be trifled with unless you know what you're doing.