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From the Apollo 13 radio transcript, as they're going through the Lunar Module activation checklist:

060:58:58 Lousma: Okay. The only item on page 10 is to deploy the landing gear.

060:59:06 Lovell: Okay, we'll do that now.

[Comm break.]

061:00:10 Haise: Okay. The landing gear are down and locked, Jack, and looking ahead now at page 11, we've done all of that.

They're clearly not planning to land anywhere, and any balance disturbance from having the gear up is dwarfed by the balance issues of having the CSM hanging off the docking port, so why didn't they skip it? They skipped a number of other non-essential steps in the procedure (such as activating the S-band antenna), so why not this one?

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According to LMA790-3-LM Apollo Operations Handbook: Lunar Module LM 10 and Subsequent Volume I, section 2.3.6:

The landing gear must be deployed before descent engine firing. If not deployed, the landing gear would be in the path of the descent engine plume and would be damaged.

The manual also mentions that the landing gear is deployed with explosive charges. I would imagine that firing an engine near the unexploded charges would be a bad idea.

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    $\begingroup$ Nice! Clear the engine path. $\endgroup$
    – Organic Marble
    May 2 '19 at 3:21
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    $\begingroup$ Heat on the landing gear from the rocket exhaust could travel up the gear legs to the LEM structure, not something you'd want! $\endgroup$
    – GdD
    May 2 '19 at 7:32

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