And who pays for the Accounting firms to train and upskill their staff in Xero?  Would you rather that clients were charged for that time directly?

It could be argued that their margins already include those costs.

There is no argument, they mostly work off margins of between 70 and 85%.They partly justify those margins on training (and rightly so to a degree) but mostly on office and IT overheads. But Xero provides everything for free! 
A highly experienced (8 years +) Senior Accountant average salary rate per hour is $40 an hour which on average is charged out at $200 an hour

I get the impression from the wording in your post you may have inside knowledge of an Accounting Firm? (sorry ajobbins for ripping off/paraphrasing your question). 
Becuase its different to the experience i saw when working for one of the top tier firms a few years back.  While I'm sure many partners/managers would like to have staff 100% chargeable there is a great deal of time lost to training and continuing education - getting CA qualified for example.