My use of "buzz by them" was obviously not a hit in this thread, and I guess I didn't use the right words. I'm not out trying to "buzz" anyone, I always slow down and try to assess each pass separately and do it as safely as possible. Some might say the rule is I need to give a verbal alert each and every time, but I'm sorry, in practice that's not always the safest thing to do. I think the OP explained pretty clearly why...you can startle people and you never know what they're going to do.
I've felt the same way on multi-use paths and even on bike paths. sometimes yelling "on your left" freaks people out and they end up moving left into your path rather than staying right. after several close calls I've gotten into the habit of slowing and riding by people if there is plenty of room rather than taking a chance. if it's another bike I almost always anounce myself.
as for whether or not to use multi-use paths for training...we don't all have a choice. if you live in an urban setting there isn't always time to drive 45 minutes just to start your ride. I use paths but only during down times, like mornings during the work week, never on the weekend.