I'm sure there's nothing here that you haven't heard before and that you haven't thought of but here goes anyway.

Northwest has pretty good elite benefits for domestic travel (unlimited free upgrades when available) but very poor elite benefits for International travel and Hawaii travel. You may want to consider your travel patterns and see whether any of the other airlines would provide better benefits for your plans.

Otherwise, as you said, it's a matter of cost vs. benefits. How much extra would it cost for the benefits that Elite status would get you? If minimum cost is the most important thing then buy your tickets through Priceline and forget the FF benefits. If convenience, FF miles, and occasionally sitting up front are more important than find an airline (or more correctly an alliance) and stick with it.