The purpose of the Libertarian party is to split the conservative vote. They dont want the LP to be available as a protest vote for moderate Dems.
While he doubt would be the choice of some Ds, and maybe a good portion of the Bernie Bros holding their nose, the vast majority of Ls are disaffected Republicans who are pissed that the Rs say all this small government stuff, and accomplish nothing.
I think Amash dropping out is good news for Trump. (despite what the hordes on Twitter say)