> unless you have a solid plan to fully utilize it for the full length of the purchase

Hum... I'd say it's much more reasonable to look at the ROI. Making investments to supply peak demand or to hedge against rare risks is perfectly ok.

Fair point, I guess the point is that unallocated resources - space/power/servers etc. can become huge stealth money sinks, eating budget every hour of every day. Being cognizant of the consumption economics before you stump up for resources is important, as is fitting the investment model to match those economics. Setting utilization/allocation targets are just one way of measuring if those models efficiently match. This is true for any service or resource consumed.