Precisely! I use displacement for close ups and for shots where the displaced surfaces would be visible in silhouette. These are the to conditions where bump mapping and normal mapping fail.

You can get some performance gain by using mental ray approximation nodes at render time as they will adjust the tessellation dynamically.

Another option for this problem is to model the text as separate objects using the bevel plus tool but that would add a lot of geometry, especially in the case of legos where there could theoretically be lots of blocks.

This is primarily an exercise to understand displacement mapping. I personally prefer creating the geometry myself. However, I am rarely time constrained on my projects. If the schedule was tight and the additional render time was acceptable displacement is an option.