It makes some sense for Lindsay to be called on to testify at a homicide trial, even if the event happened years ago. If she were about 17 when it happened and she's now about 30ish, then it happened in about 1993 - lots of advances in DNA etc. since then. Perhaps new evidence has come to light since it happened due to advances in technology (maybe the murderer was never caught or actually convicted due to lack of evidence at the time?)
Several years ago, I was hunted down by the FBI and called upon to testify at a homicide trial a full 10 years after a murder occured (but the killer was never caught the first time around) - involving two people that I had briefly met while in California, 14 years before. I didn't even know anything, I never saw a murder happen, I barely knew these people, except to unfortunately witness one random violent scene between them & had called the police, so my name was on record. New technology had allowed new evidence to come to light, allowing them to put the killer on trial years later, & I was simply called upon to retell what I remembered seeing 14 years earlier. So if Lindsay had actually been the sole survivor of a multiple homicide, she would definitely be needed as a key witness, no matter how long ago the event occured. Her testimony would be crucial and mandantory. I'm glad they did something like this for the storyline, it pulls all the loose ends together.
And with the little hint in "Raising Shane" that they're good friends outside the job as well (Danny knew things about Sheldon that neither Mac nor Stella knew
