One would assume that it's a safe bet it's common in most other systems as well...
I guess if one was ignorant of the past 300 years of science one might do that. Otherwise, it would be too obviously stupid, as it would require believing something trivially and completely false: that what we assume is particularly likely to be true.
Why not just assume the sun moves around the Earth? It's obvious, isn't it?
In the present case, there is a whole bunch of stuff to be interested in.
1) There is always the possibility that the chemical environment or formation process of the Earth or solar system was anomalous in some way. For example, it has us in it, and as near as we can tell intelligence of the specifically human, universally representational, machine-building kind is fairly rare (there is no evidence for it elsewhere.) So given that, it is not implausible that there are other weird things about our solar system, and we should likely be cautious about assuming that other planetary systems are much like ours. The astonishing discovery of hot Jupiters, for example, is an instance where we were looking for something that we were almost certain didn't exist (simply because it was the only place our current instruments were sensitive) and found something, quite unexpectedly.
2) Even given that water is common (which we don't know until we've measured it) there is the possibility that it is almost always sequestered in dense, cloudy atmospheres, or in icy outer planets, or cometary halos, etc.
3) Even given that clear atmospheres exist (which we didn't know until these guys measured it) we don't know what their typical composition is (and we still don't, based on a population of one.)
4) Even given that clear atmospheres have water (which we now know) we are most interested in finding Earth-like planets, which means a clear atmosphere with water and oxygen (which is a key signature for life as we know it). Testing out various detection ideas and proving they work is a huge step forward even if the first planet they found has a hydrogen atmosphere.
So there, just off the top of my head, are a few reasons. Assumptions don't produce knowledge, which is why we shouldn't give them much credence. Observations do produce knowledge, which is why we should be excited about a new mode of observation finally bearing fruit.