Do speakers need to be enclosed?

My friend recently went nuts with a 8 channel power amp and has ghetto speakers just thrown up on his dash, velcro'd to seats/rear deck and even a few hung from the roof by zip ties

Yeah...

Anyways, His speakers are fully exposed. It brought up a thought to me, Do they need to be enclosed in some sort of enclosement?

2 of his speakers are half way exposed dipping through some foam padding on his back seat...

And yes it sounds like garbage,

1: because all the speakers cost no more than $15

2: Because they are always falling down and rattling against each other and crap