A friend of mine, David Delitta, shot and killed by a robber as he attempted to draw his gun from his ankle holster.
http://www.odmp.org/officer.php?oid=3998 As in David's case, I would think one's first reaction would be to go for your gun. If it is in a cumbersome position as in an ankle mount, tragedy can obviously occur. I would think that it would require extensive training and practice in drawing effectively and firing from the kneeling position before an individual used an ankle holster.
Just a comment on my limited knowledge of ankle holsters.