A spare lander is a good investment if you do a lot of mining far away from Sol and dislike saving all the time and reloading every time you make a mistake; after all, the cost of a lander is equivalent to 25 fuel, which is usually less than the fuel you need to get back to Sol, get a new lander and get back to where you were.
You can only use one lander at a time, so spare landers are unnecessary if you, when you lose a lander, always go back to a save from before landing, just like Michael said. UQM lets you decide how often you save and load, which allows you to control how difficult the game is. You can get away with a lot less equipment and mining much more hazardous planets if you keep reloading until you get what you want instead of accepting that you lost a ship or lander or something.
Personally, I like the challenge of completing the game without "undoing" my mistakes. I also avoid using notes from previous times I've played the game. There's nothing quite like accidentally bumbling into a hostile homeworld with 0 crew on your flagship and no saved game (I actually survived that one; it's all about activating the emergency escape warp at the right time).
Yeah, that's something I never get with the "I'm going to save every time there's a problem and reload until I get what I want" crowd. It takes the difficulty out of the game because it removes the consequences of your actions out of the game.