If the plan has a sophisticated recordkeeper that is tracking eligibility AND deferral elections then obviously it is doable. Even better if there is a 360 bridge. But the reality is many employers do not have that kind of capability available to them. So for example, there might be a recordkeeper, but they may not be tracking eligibility because the requisite data to do so may not be exportable from payroll in a useable format. Without full censsu data the RK cannot track eligibility and if you can't track eligibility than the RK cannot track auto enrollment.
Oh and even if the RK can track eligibility and deferrals the employer has to be super meticulous about always checking the RK for any deferral elections that were made (because emails get lost, missed and diverted to junk). That's really hard to do if you have 15 employees (perhaps 5 contributers?) and one change every other year.
Bottom line is all but those with the most robust reporting capabilities really must use the 10% approach because the reality is they cannot track auto enrollment/increases on their own (i speak in general terms obviously some people can do this). And I promise my description of the technology requirements above excludes a LOT of plan sponsors. ergo a LOT of employers should be using this 10% approach.