I've just been testing fragmentation on a table that has a clustered index on a guid (ouch!), not my design. But I've found that with an appropriate fill factor, in this case 90%, adding 100,000 rows to a table of 8 million fragments that index just a fraction of a percent.

I conclude that page splits can be minimized with an appropriate fill factor. I also defrag all indexes that are more than 10% fragmented on a weekly basis.

Too often I think the clustering on the identity column is done out of laziness - not spending the time to analyze the queries hittng the table to determine the best choice for clustering.