The use for memory athletes is mentioned, the difference between 47, 047 and 0047 can be important to them.
Now, let’s assume a phone number.
0123 0456 78
If you neglect the first zeros, you will end up with three images
123 456 78
Now, where do you put the zeros?
123 456 0078 ?
0123 456 078 ?
Surely a system can help, in which case you substitute a zero by a logical system. For example, for a 10-digit phone number you can create a 4-4-2 system. So the first image is 4 digits, the second is also 4, the third is 2.
Then of course it also depends on the kind of system. If it is a 2-digit PA for example, your image for 0407 could become the same as your image for 0047, so you would require a format that distinguishes the two. For a single part major system this problem might be minimal, but the format needs to be clear.
A big point to mind is a string that breaks your system. What if your system is for a 4-digit number but you need to remember a 3-digit number? Will you be able to distinguish 047 from 0047?
Now, your argument for the number lines in competition stands, if your system remains clear. If each image represents 4 digits, any image representing less digits would be preceded by zeros. Clear rule, and it can work.
What you might increase is repetition. a four digit major system, with additional 3-, 2- and 1- digit images will give you 11.110 images to know. if you remove everything preceded by a 0, that will allow you to lose 1110 images. Which in turn means that 11.1% of your images will have multiple meanings to them, and context will have to prove which meaning is the one you need.
In the end, every system has benefits and downsides. You will have to find a system that works best for you.