As it looks above I would say simply that knowing how many people want something (I assume you mean demand in the economic sense) enables you to calculate revenue, costs and all that based on those jolly principles of accounting. Comparing some relavent feature is, um.. Comparing some relavent feature and no more.
And the exchange value is determined by various incalculable factors such as 'simple desire', ability to exchange etc etc.
We couldnt without knowing 'live' every one of the above determinents for every person - and hence we shouldn't even try to plan economies.
It may be that how you regard exchange be influenced by aspects of self-identity.
The LTV serves as a measure on the production side of commodity exchange.
The STV (Subjective Theory of Value) along with variants such as "Laws of Diminishing Returns" attempt to measure desire, primarily from the consumer end.
The distinction may also be relflected along a continuum from "Doing" to "Having" in terms of self-identity.
If one watches the "Lifestyles of the Rich and Famous" - identification of the protagonists is almost invariably in terms of possessions, ownership, ie. Having. If it is ever Doing, it is in the context of vacationing or consumption - Never Production.
On the other hand, amongst the working poor, self-identification is more likely to be in terms of Doing. "I'm a plumber" "secretary", "gardener", "teacher", etc. It is generallly from the Production side.
There is no "Cosmic Truth here. The LTV is better suited to the objective world of social relations. It seems to provide an objective description of surplus labor - and by extension - certain psychological formations, amongst them alienation. There are aspects of the subjective that are also important and have to be more thoroughly understood. I Firmly do NOT believe in any "a priori
knowledge—that is, knowledge independent of material reality..." as proposed by Carl Menger