This quote comes from the monster entry for the common mule in Moldvay Basic (page B39):

If the DM permits it, mules can be taken into dungeons. A mule can carry a normal load of 2000 coins (or 4000 coins at most, with its move reduced to 60’/turn).

That’s 200-400 extra pounds of equipment or treasure.

Some other benefits:

• No XP sponge (the mule does not get a share of XP).
• Minimal chance of theft or rebellion (unlike some retainers).
• Monster detection system. Presumably one could get the same benefit from a trained war dog, but a mule might be less likely to die in combat partway through the expedition, since it is probably not going to be an active aggressor.
• Mules are still used by the US Air Force in Afghanistan.

I would say that mules compare favorably to hired porters.

Unlike most other game components, mules are primarily about encumbrance. Their function is to carry things. If you are not using encumbrance rules, don’t be surprised if your players ignore mules. I have never been satisfied with the old coin-based encumbrance system, or more “realistic” systems that sum weight carried (realistic is in scare quotes because such sums don’t take into consideration awkward items or how the weight is distributed, which is just as, if not more, important than the absolute quantity carried). Luckily, the LotFP encumbrance system (free Rules & Magic book, pages 38-40) covers mounts and pack animals as well, though there is no dedicated encumbrance record sheet for animals yet (something I hope to rectify soon — it’s on my list of things to do).

Dragon #48 (“Carrying a heavy load? Let a mule do it for You!”) suggests that mule training should require time and resources:

Players can train mules in uninhabited caves and ruined fortifications, offending their sensibilities until the animals are used to odd smells, dank dungeons, and strange noises. All of this takes time — up to several months if you want a really good mule — but the players can hire someone else to do the job so that they remain free to go adventuring while the mule is being trained.

The article goes on to say that a druid could do this through magic more quickly. As long as an adventuring party is not asking their mule to do anything ridiculous, I don’t think I would require special training.

If that’s not enough, you can go read the 121 post (at the time of this writing) Dragonsfoot thread on mules, from which some of these observations were drawn.

Image from Wikipedia