That's not the only thing you have to worry about, either. Grunts can temporarily knock out a walker with an electromagnetic pulse grenade, or else use active camouflage (a personal cloaking device) to run beneath it and plant explosives in its knee joints or toss grenades and explosives into an exposed vent. This should give infantry a good chance against a lone walker, which is why effective tactics will require vehicles and infantry to support one another, just like they do in real life. And as you can see, the rock-paper-scissors balancing is something the designers are working to maintain, especially after the experiences with Battlefield 2, where some of the vehicles sometimes were a bit too powerful.