At the very least, spinning off individual businesses prevents self-preferencing. Google can right now leverage its dominance in one area to increase market share in another. For example, if I load my GMail account in a browser other than Chrome, Google will "helpfully" suggest that I change my default browser to the "recommended" Chrome. This behavior makes it harder for upstarts to get a foot in the door across a wide range of products — by removing these synergies we reduce the grip Google has across all its lines of business.

I suspect the AT&T example may be more similar to the current situation than you're thinking. AT&T wasn't just the network, they also manufactured the phones themselves through subsidiary Western Electric. They leveraged their monopoly in phone service to drive customers toward leasing their phones, similarly (if more aggressively) to how Google drives customers from one product to another. Whether this remedy will be so far-reaching beyond search I don't know, but in the abstract there would be benefits to splitting up the conglomerate.