English, as a global language, has unavoidably a lot of varieties. When we learn English do we really have to stick to one specific variety ? If yes why and which?
Great question. Yes, English has many varieties, but as far as I know only two of them are normally taught in a classroom context: British English (assuming Received Pronunciation) and American English (assuming General American).
To make a decision, ask yourself:
Which version will be more useful to me? (Professionally, travel-wise, etc.)
Which version am I likely to more easily imitate? (Maybe you have a knack for RP, but not GenAm.)
Which version will I hear more often?
Which version sounds more appealing to me? (Some learners feel that RP sounds more "prestigious" or "proper", etc.)
Note that whichever version you choose, you will be understood by most English speakers around the world, despite the pronunciation and speech pattern differences.
However, let me offer several pragmatic advantages of going with GenAm:
GenAm dominates the media. You'll have no shortage of movies, shows, etc., and those are important (and entertaining) learning aids.
GenAm dominates the internet. Think YouTube, podcasts, lectures, etc.
This is a bit of a guesstimate, but there are many orders of magnitude more speakers of General American than those of RP. According to Wikipedia, RP is spoken by roughly 3% of the British population. By contrast, General American (or its close approximation) is very common in the US.
So for example, if your job puts you in a position to communicate with a British person, chances are, this won't help your RP very much. But if the guy or gal is American, they'll probably be speaking GenAm.
To answer your other question: No, you don't have to stick with a specific variety. It's up to you. Many people seem to mix and match and feel just fine :) Just make sure you can understand both General American and RP and you'll be good to go.
The Transatlantic version was popular some decades ago: a rhotic mix of American and British pronunciation without any regional nonsense and with emphasized clarity.
Today, the American Theater Standard is most likely what one should aim for. It is rhotic. It is free of drawn-out vowels. It is very light on nasal sounds. No affectation. No Eastern Seaboard, Brooklyn-inspired sing-song tones. No mouth-filled-with-marbles that today passes for genuine British talk. No Cockney sound-chewing, no Irish brogue or west country lilt. In other words, it is clean, sensible, and, when used correctly, quite beautiful.