What is the significance of root and page level fragmentation in a clustered b-tree index?
Fragmentation in non-leaf levels is typically not an issue due to the difference in the number of pages between the leaf and non-leaf levels.  For example, here are the number of pages per level in a fairly ordinary 8 million row table with an int primary key:

index_level page_count  
0           156623          
1           653             
2           5               
3           1               


The 659 non-leaf pages have a very good chance to remain in buffer pool because any seek into the table starts at the root and walks down the tree.  Even if they did need to be pulled from disk and it was completely random 8k I/O, they still make up less than one half of one percent of the table.