It all depends on the map. Most have choke points that, during the infantry phase can slow the advancement of your opponent(s). Often, your opponent(s) will also try to take cover in nearby buildings, mining the entryways can chop them into pieces when they seek shelter.
Once the heavy vehicles roll out, though, it becomes a question of whether its worth the resources to deploy them or not. Often times, your resources are better spent countering tanks directly or bringing anti-infantry vehicles into play.
It also depends on your faction choice. Mines are a better option for the Brits, because they are attuned to a more turtled style of play. Panzer Elite, by contrast, is a highly mobile military and using too many mines can actually be a detriment as you try to maneuver around the battlefield.