Fairy Tail Ep. 171

[Buffalobiian]

You keep Yukino as the backup because she's employed by you and isn't readily going to run away.

Lucy isn't readily about to serve the "game masters", so assuming they want both a backup and a sample, then it makes sense to use Lucy right away while Yukino is stashed aside for later in order to lower your risk of losing these precious celestial wizards.

Plus, she's the main character.

[DarthEnderX]

Lucy isn't readily about to serve the "game masters", so assuming they want both a backup and a sample, then it makes sense to use Lucy right away while Yukino is stashed aside for later in order to lower your risk of losing these precious celestial wizards.

Perhaps, but by using an actual willing participant you are able to sidestep a couple of fairly enormous hurdles. Like, her resisting. Her trying to escape. Her insanely powerful friends coming and kicking your ass. etc.