I will say it again: Preparing for an exam in college, HS, is not what we are against. No one I know who is successful takes an exam cold. I am a 50 yo research scientist. Everything I have done in my career is based on preparation.
Using the spelling bee example brought up: there is a difference in learning word origins, and how to spell many words, compared with obtaining the limited word list by the test organizer, and memorizing that. The latter is what we are complaining about.
The CogAT's purpose is to:
To assess students’ abilities in reasoning and problem solving using verbal, quantitative, and nonverbal (spatial) symbols
This is from
http://www.riversidepublishing.com/products/cogAt/
They are trying to assess the ability to think and solve problems. Using practice exams may improve the score =, but it does not improve the ability to reason. Chess will improve the ability to reason. Reading, imagination, playing music, all fire those neurons. Practice CogAT's may (or may not) improve the score, but they will not improve the root intelligence. Since the county is going for a measure of the root intelligence, improving the score without improving what is being measured is called gaming the system.
I suppose if your child had a WISC scheduled, you would prep for that. I do know how to improve ones score in the WISC. It is possible, and is considered unethical.
I am sure you will pick that apart.