Though these two men in 1984 are in some sense opposites—Syme is brilliant and Parsons is perhaps the least intelligent person Winston knows—they have in common their unquestioning allegiance to the Party. Yet both men are destroyed. Syme disappears, having been "vaporized," an occurrence Winston predicted. And Parsons is arrested as a result of having been overheard in his sleep saying heretical things against the Party.
Orwell's overall point is that in a totalitarian society, absolutely no one is safe. Though Syme genuinely believes in Party doctrine, he's too intelligent for his own good. The Party has to get rid of such people. The quote I have chosen, in which he describes the destruction of words as a "beautiful thing," is significant because it shows Syme as a true believer. He's so committed that he speaks in an almost poetic tone about an action—the destruction of the English language as we know it—that most of us would be appalled by were we to actually see it happening.
It's ironic that Syme should express himself this way, since we would expect a kind of machine-like assent by someone like him, rather than his dreamy waxing rhapsodic over the Party's plan to make intelligent thought impossible by eliminating the words in which it might be expressed.
If his own intelligence is Syme's downfall, then we, like Winston, would assume that if anyone would be safe, it would be the unthinking and dull-witted Parsons. But by showing Parsons too as a victim, Orwell seems to make the point that the unthinking allegiance Parsons appears to stand for is an impossibility. Even the most subservient person harbors resentments that can emerge when, as in this case, he's overheard talking in his sleep.
Yet to the end, Parsons does not understand what has happened to him. When, "almost blubbering," he blurts out the single word "Thoughtcrime!" in answer to Winston, he appears to be in a state of shock. More than anything, his manner conveys that his loyalty was not a kind of ruse or a façade to cover a spirit of rebellion.
So, it's not merely those who feign allegiance who will be discovered by the regime. Those who honestly profess loyalty will be victimized just the same. The seeming polar opposites of Syme and Parsons are destroyed essentially for the same reason, though the actual circumstances of Syme's arrest are not shown to us, beyond Winston's understanding that the Party does not want people who are smart and can think for themselves and must therefore "vaporize" them.