Here are my 2 cents.
I have grown for 14 years. I helped at an orchid nursery for 10.
Here is what I have been told
Urea needs to be broken down into a useable form of nitrogen. This is done by bacteria that typically needs soil to live. So, it goes from urea to ammonia then to nitrite and then nitrate(I may have gotten the order wrong) Nitrate nitrogen can instantly be used by plants.
The arguement for urea free is that the plant has the higher concentrate of nitrate nitrogen that can be used right away.
Supposedly, urea will stay in the medium and "turn" it quicker because of the build up of fert that needs to be broken down.
But, some say urea will hurt the plants. I have never seen that happen. I personally believe the urea form is pretty much useless to the plant.
Now, I only use a high nitrogen fertilizer once or twice a year anyway. Also, I use it in combo with a high magnesium fertilizer as well as some other trace elements which help the plant use the nitrogen more effeciently.
The problems with using a high nitrogen fertilizer is that it can make plants grow "leggy" and flower less as well as having a lower flower count.
OK, I am not a scientist and this is all just from what I have heard, read and seen as well as what I could remember. I do know a lady that never fertilizes and waters all her collection with lake water which has enough nutrients to make her collection grow and bloom wonderfully.