Actually the VC/NVA used to attach mines to small chutes
retrieved from illum mortar rds and hang them in trees around
LZs... let the U.S. Hueys come in and the upwind would carry it
towards the helo..BAM!! But most of the time this crude anti-helo
mine didn't work well... 
As for AH going to LZs, AHs may have to run suppressing fire if the
LZ is hot...AHMs can prevent them from doing so, therefore putting
them at risk from other types of AA fire...handheld SAMs, AA guns,
etc...
AHMs, when activated, travel at high enough speeds, giving the
pilot(s) little time to take evasive action...Imagine leh, the
Russian AHM is pretty cheap at about $6000....compared to a
multimillion chopper...even worse if its a UH, considering they are
slower and carry troops...how to heli-repel like that? 