L4 and L5 are stable and therefore tend to collect space junk - this is one reason why you would pick an unstable point (like L1, L2 or L3).
Stable points are at the bottom of the proverbial energy well, and it would be harder to leave them then to leave an unstable point, which is "perched" at the top of the proverbial "energy hill".
IBM 760XD
"Next Generation Laptop" - Windows 2000IBM A31p
Some reasons to pick an unstable point: