I can understand the feeling of being threatened by in-your-face religious zealotry in public places. Most of the time they are just dull/annoying (especially the ones that talk about going to hell or praying for God to save us heathens).
I can too but I'd also shrug it off pretty fast, yes I've had experiences.
I personally do not find it offensive, but I would find it scary.
I guess it is subjective to the person though. If your roommate is firmly against those belief's, it can be taken as offensive having to be subjected to hearing them screamed by a lunatic when your roommate may not have been able to get away.
I mean, when I'm waiting to get on a plane -- I don't want to hear about Christianity, I'd be pissed (even if it is a crazy person) if someone was standing there in the terminal reading passages or screaming 'Jesus is Lord' and I couldn't just simply walk away since I was waiting to get on the plane.
"My mom told me there was a weirdo on every bus, but I never could find him." Emo Phillips
I would be just as discombobulated by someone shouting and ranting about Atheism as I would Christianity...even someone shouting about the Tao of Pooh!
To be fair, I have never heard such a public atheist rant. (Possibly because I live in the suburbs.)
I think it also depends on my current mental state how well I shake it off/ how seriously I take it. The time I let it most unnerve me was one instance in which I had had a really long cloudy grey day and was waiting out in the drizzle for the bus because I just wanted to get home. This man pulled up to the large throng of students waiting for the bus on his bike, and proceeded to get on a soapbox (a metal mesh one) and passionately declare about my salvation and to accept the spirit of God into my heart. If I hadn't been in a fairly familiar environment with fairly familiar people I'm not sure I'd have taken it very well.