My position is very much in line with Jordan Breen's thoughts on the subject.  To wit:

It's my personal opinion that given the landscape of women's MMA right now, it isn't necessarily prudent to produce rankings. Beyond the pronounced fracturing of the talent pool and the fact it undermines the ability for top women to fight each other, there are severe logistical problems, the chiefest of which is weight divisions. Although any rankings Sherdog.com publish will be based on MMA's authentic weight classes under the Unified Rules, women's MMA hasn't exactly adhered to them rigorously thus far.

More than ProElite's inane weight classes, Japanese MMA, where a huge bulk of top female fighters ply their trade, is all-over-the-map with its weight classes. One of the world's best women, Satoko Shinashi, fights in Deep's 99-pound class. 58 kilograms, or 128 pounds, is often used as a weight division in Japan, as well.

On top of arguably the biggest market for women's MMA often employing garbled weight classes, there's presently a disproportionate amount of women's bouts in MMA that take place at catch-weights, as many women have yet to find their most suitable weight class, or because of the other most pressing issue: depth.

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You're spot on in the assessment that rankings -- especially for developing realms like the sub-lightweight divisions and women's MMA -- can be seriously influential in mobilizing promoters. However, the raison d'etre for rankings should be primarily to offer a picture as to whom the sport's most objectively accomplished and meritorious fighters are. Its ability to put ideas in the heads of fans and promoters are a fantastic side effect, but should never be confused with their purpose.

Certainly, at some point female mixed martial artists deserve to be ranked like their male counterparts.  Breen highlights several key difficulties which make the ranking of women nearly impossible if your goal is, in fact, to develop an honest pecking order of the top fighters.  This is not any sort of indictment of female MMA; it's merely the reality of the situation.

Two conditions need to be met before any type of quality rankings can be produced.  First, the pool of female talent must deepen.  This is very much a subjective measure, because it would be hard to produce a definitive number of necessary female competitors.  The fact that some women are inclined to often float between weight classes and catch-weight bouts are quite frequent indicates to me that there aren't enough top-level competitors - that have been identified as such - to produce an accurate set of rankings.

Secondly, there must be much more adherence to a set of weight classes across promotions.  It's difficult to know which fighters are best at any particular weight class when one promotion emphasizes say 140 pounds as opposed to a more traditional 145 pounds.  You could conceivably put these two classes together in a set of rankings, but it will be at the expense of accuracy.  Weight does matter, especially when one is attempting to identify the top fighters at a given size.

Rankings should be reflective of the athletic accomplishments of those being ranked. If the product is merely a popularity contest or guesswork is too big a part of the process, the results are basically worthless.  It goes back to development.  Female MMA isn't at a point where rankings can be made meaningful, in my opinion.  This is not necessarily a bad thing.  The environment is simply too unsettled, and the infrastructure has yet to be fully constructed.