It's not the outer hull, it's the inner mechanisms, power conduits and so forth. Not merely causing the leg to cease functioning, it's probably quite possible to create an explosion that topples the Titan, or causes the power across the entire machine to short out.
Even just causing one leg to freeze up mid-stride could send the entire thing falling over, and the sheer kinetic impact of hitting the ground could deal additional severe damage if it doesn't just cripple the entire thing due to being flat on its face and unable to get back up.
Seriously.. ???
Do you honestly think that a stray plasma/melta shot down a entrance corridor is even close to inflict that sort of damage?
We are talking about something up to 30 meters tall. A walking building. If it would easily be dismantled by hand held weapons,
then it would not be that big of a deal to start with.

And Plasma guns meanwhile are so cheap that you can give them to regular humans.
Considering the price of not blocking a boarder on their way to the princip,
then its absolutely worth it to store a healthy supply of plasma weapons.