Those who've JB'd their phones and run SBSettings can tell you that shutting down apps from the multitasking bar absolutely DOES free memory, because we have an easily accessible tool that tells us how much is in use vs. available at any point. Close the multitasked apps and watch your available memory climb. Obviously, doing things like shutting down unneeded Safari windows reclaims even more.
I've not experienced the freeze described, but I'm also still on 4.0. Personally, I routinely shut apps down as much for convenience in accessing those I'm actively multitasking with as for reclaiming memory.