Well - they spread disease. As to the yellowing/browning of the
leaves, could it be chlorosis? Although Ceanothus are lime-tolerant,
apparently they are prone to chlorosis on very alkaline soils. My C.
'Concha' has yellowing leaves, and it didn't even knock itself out with
its floral display this year. I've given it a watering with a chemical
feed that contains sequestered iron and other stuff that gets locked up
in an alkaline soil. I had another 'Concha' that simply turned brown
and died for no apparent reason.