Minecraft – What Happens if You Destroy a Dungeon Chest Without Loot?

I've done some number crunching and I think I have your answer for you. These numbers assume that (a) you use the mining method in the accepted answer you link, (b) that the ores are uniformly distributed, and (c) that you're running a version of Minecraft between 1.6.0 and 1.7.3 (the chart you link hasn't been updated for 1.8 yet).

Mining for an hour with ~30 iron picks should yield you:

• 7,201 cobblestone
• 77 coal
• 308 redstone
• 46 iron
• 8 gold
• 8 diamond
• 37 lapis lazuli

Similarly, using ~6 diamond picks should get:

• 9,166 cobblestone
• 98 coal
• 392 redstone
• 59 iron
• 10 gold
• 10 diamond
• 47 lapis lazuli

As you can see, the iron and diamond yields are far below what goes into the picks. In other worlds, through solid stone, you operate at a loss. Caves make an enormous difference when it comes to mining efficiency.

(All this, of course, assumes my math is correct. Feel free to double-check my work in the spreadsheet.)

The same rules apply to Monster Spawners as to the natural spawning of the mob in question, with two exceptions:

1. Ordinarily, hostile mobs will never spawn next to the player, but with spawners they can.
2. Natural spawns require ground underneath the mob, but Monster Spawners can create monsters in midair.

It is also important to understand the effect of light level: what is relevant is not the light at the spawner but the light at the location where the mob spawns, so putting a torch on the spawner is just a way to center the light, not inherently good — and is in fact inadequate to light up the full spawning range to level 8, though it happens to be sufficient for many natural dungeons. The actual requirement for safety is that the full spawning area (8×8 × 3 high box centered on the spawner's low-coordinate corner) is sufficiently lit (or does not have valid locations to spawn, e.g. is filled solid).

In your case, where you have two spawners right next to each other, there is no way to spawn just the skeletons except by lighting or filling up with blocks the entire spawn volume of the cave spider spawner, leaving you with just a thin slice of skeleton spawner volume (which will therefore have lower spawning rates).

Another possibility would be to construct a sorting system which separates the skeletons from the cave spiders, but I don't know of a particular design to suggest. Cave spiders are particularly difficult to handle because of their small size and ability to climb.