That is a question sure to invoke a variety of heated responses. Short answer (in my opinion). Urea not bad. But, urea probably not best, either. You won't kill your orchids fertilizing with a urea based fertilizer. They may not be getting quite as much nitrogen as you intend. Short reason is that urea is typically broken down into plant munchable bits by soil bacteria. You have few soil bacteria in your potting mix. So, the prevailing wisdom is that since plants can't utilize urea directly, you are wasting your time fertilizing with it. That said, it isn't at all true. You can grow good orchids with a urea based fertilizer. But you have to use a lot more nitrogen to get the same result. Most of the people I know have switched to non-urea based fertilizers for that reason, but any fertilizer is better than no fertilizer. So, don't worry about killing your plants with the urea, but you may want to consider switching at some point.
Rob Halgren, MSU