The value of bench dips in preparing beginners depends greatly upon the technique with which they intend to perform the full dips, for which there tends to be great variation.
In the most general terms, the joint actions of the two exercises would appear to be the same—shoulder flexion and adduction, and elbow extension—but bench dips have a limitation that alters their mechanics entirely: bench dips are a closed chain movement. The posture of the shoulder is therefore tied to the position of the feet. Furthermore, the bench itself presents a physical barrier which governs our degree of freedom. The net result is that our centre of mass is forward of the position of the hands, and the force vector of our effort must therefore contain a significant horizontal component, pulling our mass back towards the bench. This can be minimised by sitting close to the bench, but it cannot be eliminated. And the bench precludes the retraction of our shoulders.
Bench dips are consequently dominated entirely by our triceps brachii and anterior deltoid, with the pectoralis major, pectoctoralis minor, serratus anterior, and the entire core and posterior chain rendered ineffective. And we can conclude, therefore, that they would have limited utility in preparing beginners for full dips.
Such a conclusion assumes, of course, that our full dips are performed in a manner in which our centre of mass passes through the line of the chest and shoulders, that our movement contains significant components of shoulder retraction/protraction and abduction/adduction, and that our body is held straight and firm. And in considering common variations of the dip, this is not a good assumption. The two lifters below, for example, illustrate a posture typical of gym-goers characterised by protracted shoulders, spinal kyphosis, and a lack of support and control in the torso and extremities. And the bench dip might therefore be considered a rather good approximation of their dips.
But if we accept that, we are ignoring so many of the benefits of the dip as an exercise, which include development of the chest, lats, ‘core’, and entire posterior chain (especially the erector spinae, gluteals, and hamstrings). A better example of dip technique is illustrated below, observing all of the postural and movement cues that maximise the benefits of the exercise.
Thus, a better choice of exercise for preparing beginners for full dips would be push-ups, since they incorporate all of same the postural and movement mechanics. For particularly unconditioned individuals, push-ups can be performed from an adjustable-height platform such as a Smith machine, thereby allowing them to follow the same recruitment patterns with some trigonometrically-governed fraction of their body weight.
I hope that helps.