I found this comment interesting:
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Delta Connection carriers’ adaptable fleets, with a mixture of seats (30, 32, 37, 40, 50, 66, 70) also offer network planners more precise tools to supplement markets during off-peak times...
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It also allows them to manipulate the supply/demand curve, and thus the price point for any particular market segment. Personally, I think the price should reflect the level of service on any given route; you shouldn't have to pay the same price for a seat in a noisy, cramped turboprop as you would for one on an RJ or a 757.