If you use a cheap tstat you can have many issues. Using a good one like the Herpstat line really minimizes the risk involved.

The problem with a dimmer is they do not respond to the environment they are in. Like a few weeks back when my a/c had its free-on go out. The temp in my home went up but my tstats knew that and adjusted accordingly. A dimmer will not adjust and would run to hot. If I was away for a few days when this happened my tstat would handle the situation when a dimmer could do alot of damage to my collection.

Sure they can be used in a pinch but in no way are better than a good tstat.