Stack and Reach: Why It's Your Best Friends

I don't understand how the fact that kids grow quickly, means that comparing two bikes is irrelevant or unnecessary:

• his current bike is too big;
  
• according to Isla, both the 24 and the 26 will fit him;
  
• the 26 looks similar in size to his current bike.
  
  

Clearly the 26 would be a better buy, as he will grow out of it more slowly, but not if it is too big for him now.

If stack & reach isn't the best method of comparing two bikes, then what is?