That's it. Unlike the actual game, the meta-game is all about raw power (or, even the ability to powerful hits, but not relevant here).
Now, the reason Pokemon have 4 varied moves is to improve their coverage, and cover their weaknesses.
In the fast-paced meta-game (competitive battling), it is generally wiser to switch into a Pokemon that can resist an attack, and then retaliate super effectively.
Thus, switching is fairly common and staying in is not necessarily helpful, as it sometimes is in the actual games for EXP points and so on.
So, to be honest, the being locked-onto-only-one-move is not that big a problem; Even if it is, the benefits of the 50% boost are greater for the most part.
So, people tend to choose this power over being more diverse, simply because the Pokemon that switches need not only resist their moves, but also KO them instead. Nowadays, the latter situation is more common, due to the increase in the offensive spectrum of battling, as opposed to defending.
That is to say, in simple terms, people find it easier to attack than to defend.
Due to this reason, and that a 20% boost is no longer enough to overcome the opponents (highly EV trained and good item holding) Pokemon.