There was another thread on here about motor scooters (the low-horsepower, step-through kind), and I discovered that in California, you HAVE to drive them in a bike lane if one is present. That changed my perspective. Less blood boiling.
I guess you have to figure out what benefits of the bike path or bike lane are being compromised when these types of vehicles use them. Such as noise from a motor disrupting the relative nature-ish peace of a multi-use path, or someone going too fast on an MUP, or riding annoyingly aggressively around peds*. With other aspects -- like if they keep to a speed limit of 15mph on the MUP, or whatever -- I've no objections.
* Um, cuz, you know, cyclists never do that.