There's an example of at least one non-paranoid use...I'm sure there are others.
Also, if Obama's planning to institute a police state in the wake of an engineered economic collapse and gun seizure, why wouldn't he just use military equipment? Why tip his hand by orchestrating weapons and equipment purchases?
What kind of terrorist does homeland plan on combating that they think they need these? These vehicles were built to take hits from roadside IEDs and RPGs.
Every terrorism incident in this county has just been some guys that a normal police officer could have shot and killed.
As far as why Barry wouldn't just use military equipment in a hypothetical police state, I would guess that there would likely be more push back from service members about firing upon American citizens and mobilization of Army divisions would cause more public notice. By using Homeland, I'm sure the administration and his media friends would attempt to portray any opposition as paranoid crazies.
PDWs are generally understood to be short barreled (shorter than an AR carbine) easily concealed selective fire weapons. They are mostly used for protection of VIPs but can be used for close quarters battle such as building intrusion/hostage rescue.
That's at least a reason for DHS to have them, but MRAPs? I don't see it.