If using the starter as your source of force, that force has to transfer from the starter to the pulley bolt. Starter to flywheel to crank to pulley bolt. Each connection and element between the starter and bolt must be stronger than, and experience the same torsion as, the bolt in order for the bolt to break loose. Think of it as the "weakest link in the chain" analogy. There is a similar dilemma in tightening the bolt with the transmission in 5th gear. Force must tranfer from the pulley bolt down to the transmission. The transmission and each element along the line must resist the torque.
The torque experienced during this procedure is likely less than operating conditions, but my philosophy is not to put stress on an element unless it is necessary. Isolating the crank pulley and bolt allows force transfer directly between the bolt and pulley without stressing other elements.
A $60 tool to possibly avoid headaches down the road seems plenty justified & founded to me, but to each his own.