Just as some examples of what building with 3×5 cards can do:

A simple structure. Note that the slight differences in the shape of the cylinders allow them to stack just like solid blocks would. In addition, it’s possible to build high even with fairly shaky architecture.

There’s no need, however, to be content with poor architectural design. This tower is quite stable; the cards are light, but everything is balanced and supported.

It’s even possible to be a bit fancy while retaining structural strength. The bottom of this tower has fewer parts than the top, but there’s enough weight to keep it together. It survived for quite some time just sitting on the rug while I walked around.

A side benefit of building towers out of 3×5 card “blocks” is that they fall without making a tremendous racket or (assuming they’re used in a reasonable way) causing any damage. New parents playing Jenga will wake up a sleeping child when the game ends; card “blocks” fall with a sound like, well, rustling paper.

3×5 cards can also be written on to create marked blocks. That isn’t an option with when dealing with nice wood. 😉

Ultimately the neat thing about 3×5 cards isn’t that they’re ideal for any one purpose; it’s that they can be put to a multitude of uses–some of which don’t look very much like traditional applications of a card. Every time I need to prototype they’re the first tool I turn to, and they rarely prove unsuitable. I can’t say it enough: keep 3×5 cards handy. They’ll work more often than you think they will.