I thought the PSR rules were that effectively you could only lose £105m over 3 seasons? Don't think they're based on turnover at all, will be profitability if anything.
So regardless of what the loan is actually spent on, in having a shareholder loan there is a direct saving on interest costs, had that loan been bought on the 'High street', which is currently increasing (or not reducing profitability).
For example, Arsenal are saving ~£25-£30m a year in interest, which would impact their Profit for PSR purposes.
That's my understanding of PSR anyway, and is what I'd say is the drawback of allowing shareholder loans.
But you're right, whether they spend that loan on players is largely irrelevant for PSR purposes, unless new signings result in an increase in player amortisation.