Well, consider it in the context of a web browser or other generic desktop application.
If you have (say) 4G memory, and you have to process 2G, then you more or less accept that things are going to be slow. (Unless you are in some kind of embedded system, but then I assume you actually measure exactly how many bytes you need anyway...) And if it really bothers you, you would probably add more RAM to your machine and the problem will go away.
If you extend arrays by constant amount, you instead end up with a 4MB data that takes five minute to process. Your users won't like it a bit. What is worse, this problem can't be solved by adding more RAM.
If you have (say) 4G memory, and you have to process 2G, then you more or less accept that things are going to be slow. (Unless you are in some kind of embedded system, but then I assume you actually measure exactly how many bytes you need anyway...) And if it really bothers you, you would probably add more RAM to your machine and the problem will go away.
If you extend arrays by constant amount, you instead end up with a 4MB data that takes five minute to process. Your users won't like it a bit. What is worse, this problem can't be solved by adding more RAM.