Against popular belief, single engine scrapers can indeed self load...they're just super slow.
However, it's obviously not as efficient as being push loaded by a dozer since they're only front wheel drive---but if needed, a fleet of singles can make due for bit without a push cat if the conditions are right.
There are various reasons why a push cat may be down briefly (Getting serviced or a small repair) In one of those cases where it's only for a short time, you can at least keep some production going.
Dirt must have a certain compaction and consistency (not too hard or too soft) and without rocks.
They will also need a very long cut as they will only be able to cut an inch or two over a very long stretch. Oh and you MUST have very experienced operators to avoid wheel spin.