Just to chime in... I don't have a problem with cycling with fish. I've never heard of it being done with guppies but I think they would have less of a chance of surviving than a sw fish because they are already being housed in an unnatural environment for them. If cycling is done right as far as feeding, water changes, amount of fish, length of cycle, etc, it can be done without harming the fish. The comfort level is something I can't comment on b/c I am not the fish whisperer or dr. doolittle.
From an ethical standpoint, I don't have a problem with it either. Many people use feeder fish or shrimp to support their livestock (even when it's not necessary). Furthermore, many people with damsels would like nothing more than to be able to catch them just to toss them in the garbage. J/K but you get my point.
There are many reasons people cycle with fish but I think the most common is that people want some life in their tank... even when it's new. Also keep in mind that not all cycles are equal. Just tossing some cocktail shrimp or fish food does not assure that enough bacteria will be propogated to support the fish that are going to be added once the cycle is done.
I have cycled both ways and so have many here. Many of the experts still recommend damsels to cycle tanks. This is still a commonly acceptable way to cycle a tank. The fact is that of the thousands of people who view these boards, I'll bet at least half use damsels to cycle but they don't get involved in these debates.
JMO