On Bentham's criterion of propinquity, one is right to regard a distant pleasure as less valuable than an immediate one. On what grounds can the criterion be defended ?
There is the point, made in another answer, that the more distant a pleasure is, the lower the probability of its occurring, since impeding factors are more likely to intervene.
But Bentham cannot subsume 'propinquity' under probability since he includes 'certainty and uncertainty' as separate 'circumstances' in the same list. Probability is covered under that heading, and so 'propinquity' must refer to something else. (Principles of Morals and Legislation, IV.2.)
Another suggestion is that the further a pleasure is in the future, the less likely one is to be able to enjoy it. But that introduces an extra factor that Bentham implicitly excludes. Pleasure for pleasure, the immediate is to be valued over the distant : the quality or intensity of the pleasure or one's impaired future capacity to enjoy it is not relevant. Equal pleasures, differentiated only as immediate or distant, are his concern.
A suggested solution
Bentham assumes that, of two pleasures for comparison under 'propinquity', we are aware that one is immediate and the other distant. That is presupposed to the application of the criterion. What Bentham may have had in mind is that an intense pleasure immediately available - supply your own example - has no pain of waiting or delay attached to it. If I have an immediate pleasure, it tops the pleasure of equal intensity that I will have tomorrow because tomorrow's pleasure will have been preceded by the pain of waiting or delay, pain which must be taken into account in the felicific calculus.
Bentham does not elaborate on propinquity and rather leaves us to work out why it features in the calculus. I offer my answer as my best inference - no more than that - to what he meant.
II. To a person considered by himself, the value of a pleasure or pain considered by itself, will be greater or less, according to the four following circumstances:
Its certainty or uncertainty.
Its propinquity or remoteness.
III. These are the circumstances which are to be considered in estimating a pleasure or a pain considered each of them by itself. But when the value of any pleasure or pain is considered for the purpose of estimating the tendency of any act by which it is produced, there are two other circumstances to be taken into the account; these are,
1.Its fecundity, or the chance it has of being followed by sensations of the same kind: that is, pleasures, if it be a pleasure: pains, if it be a pain.
2.Its purity, or the chance it has of not being followed by sensations of the opposite kind: that is, pains, if it be a pleasure: pleasures, if it be a pain.