In my more than a decade of teaching English, there was never one student I taught who had never asked me about whether it was really necessary to learn English pronunciation. And my answer to this question is always the same: is there even one disadvantage of having a good English pronunciation?
The very reason why you learn English language is to be able to communicate with foreigners in business settings, during your travels, during a quick trip to your favorite shopping mall or understand English movies without subtitles. To put it simply, you want to understand and be understood. This can happen only within a couple of seconds; there is only one shot; there is no second chance.
Communication is a two-way process. It is only effective when you understand what other people say and vice versa. If one of the parties involved in communication is out of reached, there is a failure. More often than not, this failure comes from the inability to speak English the right way; in other words from not having a good English pronunciation.