A lot has been said (and this is one of the "eternal" topics in Linguistics I think) but I wanted to give my contribution. In your question you say that "if we think about it, maybe human communication would be better off if we have fewer languages".
I highly disagree with that, and I suppose most linguists or anyone involved in languages study would, too. And the reason is simple: that is not the solution. The perfect solution would be for people to become multilingual, and not let languages die. If that was a true solution for avoiding conflicts and wars, how would you explain that even people from the same country harm each other? It would be too simplistic to relegate the wars' cause to a matter of different languages. Languages are the key to understand its people. When you start learning a certain language, you also start understanding that country, its speakers, its culture.
About that, it has been said many times but I'll repeat it: languages and culture are intrinsically related. Or better, languages are culture. Without language, you wouldn't have culture. So when we talk about preserving, or in the worst cases "saving the salvageable", we are not simply talking about words, letters and grammar rules, but anything that is correlated to that. I'm not sure one would call this a scientific reason, but I suppose it is, beyond being an absolutely good reason.
Luckily in some cases, saying that a language dies, doesn't necessarily mean that we lose everything about it; Latin and Ancient Greek are a great example: they are dead languages (i.e. no native speakers), but the material about them is plenty. However, other languages are not so lucky. The "luckiest" endangered languages are the famous ones, but let's think about the unknown ones. They are in the worst situation.
I'm not a linguist, technically speaking, but when a language becomes extinct, it's a great loss, and not just for the nation involved but for everyone. I don't want to sound too cheesy, but when a language faces extinction, it's not a language that becomes extinct, it's a part of mankind history. Extinct languages are irreplaceable.