I completely agree, the spells will be strong and have a better chance of working if created by the one who is going to cast it. It is their own energy going into the creation, preparation, and casting of the spell, therefor making it stronger. Cast someone else's spell does not allow for your energy to be what is driving it so it has less of a chance (in my eyes), the purpose for which it was created may not have been the same or (other then the fact of the fake ones on here) they may not have put a whole lot of energy into it. Some who may not take is as seriously as others may have just sat down and made one up just to post one and even if they were serious it may not have been thought out or truly had a purpose(just words thrown together last minute).
I always go with my own but I will go to the spells and search for what I need and look to help with ideas (on starting or wording) sometimes if I am stumped.
Hope this Helps, Sorry I got a little rambly. I'm tired and it is 6 am here. I have been up all night with a migraine. But don't feel bad, most of the time they do not involve pain just everything else like sensitivity to light, etc.. and I am a night owl so to speak. Just not normally this late. :)