I think it is a misconception to think that suing a mmo file is using MORE resources...
1. When there is no data content on a line, you are using 8 bytes
2. When there is data, you are using 8 bytes+size of the content
If you are NOT using MMO, then you need to declare a text field with a size=the LARGEST content you may have, so for each line, it doesn't matter it you have content or not, you are each time using the MAXIMUM possible value.
Using MMO SAVES resources...
It can even save time, because you can read the main record WITHOUT the MMO (faster) and ONLY read the mmo when you need it...