It's a good thing to have, but not always neccesary if an optimal enclosure is used (and depending on the species). For example, Understory Enterprise does not use waterbowls for their agalychnis-species (according yo their website), and they have breed them (all but spurelli) for some time with great success.
However, it's just an easy thing to do, and it'll help a lot in case they would need it. Just keep it clean!