Being harder doesn't make it more fragile, being weaker would. A lot of people don't seem to understand some of the terms when it comes to breaking or failing parts. A material can either be hard/rigid or soft/ductile, and anywhere along the scale. Think of concrete, it is hard, it doesn't bend it crumbles and cracks. Steel isn't as hard, it bends and when it fails it's usually more of a tear than a crack/crumble/shatter.

Some materials have brittle failures (hard materials, like concrete and PLA) where they shatter/crack/break violently. Some materials will bend and tear (think of metals, ABS) in what's called a ductile failure. Ductile failures are usually preferred because they aren't as dangerous or violent, and it's more of a gradual failure than a sudden one so you can fix it when it starts to show signs of failure. Note: this does not make ductile materials stronger. PLA is actually stronger than ABS.

I personally prefer PLA, it prints much better and turns out much stronger, especially when considering layer adhesion. I'm not even talking about an I3 clone, this is on my semi enclosed solidoodle 2 with a bed moving only in the z axis and a heatbed at upwards of a hundred degrees. The ABS prints break along the layers and look downright ugly compared to the quality of the PLA.