The new arena scoring makes single high merges weigh much less while conversely also making single low merges hurt a lot more (eg: unmerged bonus units). In a way this means you stick closer to your average but people seem to be a little miffed by the changes.
If you have more total merges without using a legendary hero, that may be better for scoring than having one or two blessed heroes and two low merge units (ie: the legendary hero and bonus unit. If you've got low merges, then legendaries might help more, but blessings are limited.
Currently, anyway. The new changes seem to have punished whales in some ways so they may change it again, we'll see.