What is HMRC's thinking behind SA three line accounts?

As an accountant with sixty or so very small businesses in my client bank, I'm acutely aware of the pressure they're under on costs. To prepare three-line accounts using experience, a tally-roll calculator, a heap of receipts, details of bank payments etc, may take only 2 or 3 hours to complete, including the tax return. My fees could drop and the client would be happy. 

However, with smaller jobs I have always relied on full trading analysis (not to Balance Sheet) and comparison with previous year's figures to spot discrepancies, duplications and omissions. But then HMRC say, 'if your turnover is less than £73k, you do not need to enter detailed expenses, just the year's total'. It feels like they're saying, 'we're prepared to take a risk at this level because the possible loss of revenue is immaterial'. Basically I feel a tension between good practice, client needs and HMRC's possible enquiries.

A friend in a separate practice said he has used the three-line option when he just couldn't balance the balance sheet for lack of accurate information. To be frank, that seems sensible and practical when a very small trade is involved. But I still feel the unease of not really knowing HMRC's thinking; maybe they want us to feel like that to keep us on our toes?! Members views would be much appreciated.