Adding latent variables to these models gives us additional modeling power and have shown success in applications like POS tagging (Merialdo, 1994), speech recognition (Rabiner, 1989) and object recognition (Quattoni et al., 2004).
this comes at the cost that the resulting parameter estimation problem becomes non-convex and techniques like EM (Dempster et al., 1977) which are used to estimate the parameters can only lead to locally optimal solutions.