Playing “devil’s advocate” is a lazy way to seem intelligent because it has guaranteed success. If there is likely no such thing as an objective truth (link), i.e., something that is true regardless of time and place, then any statement or thought will have exceptions.
Any decision, observation, or conclusion is tempered by the circumstances in which they arose. Even the simple equation 2+2=4 has several assumptions, such as the unchanging nature of the numbers, that the quantities are similar and comparable in some way. Reality has constantly changing quantities and qualities – in fact, the standard weight that is used to determine the kilogram can change slightly over time (link).
But there is something seductive about playing the devil’s advocate, always taking a contrary position regardless of one’s own personal opinions on the matter. On the plus side, devil’s advocate can be a tool to more fully examine a point of view, anticipating objections or points of weakness.
However, it can easily devolve into invalidation because a flaw can always be found. Even a gram is not always a gram. If the devil’s advocate is not careful, not only can the idea be discarded (even if flawed, it undoubted holds a sliver of relative truth), but the thinker might also come be convinced that he/she is unable to come up with anything worthwhile.
Challenging an idea should be helpful, and aimed towards a greater understanding – of the assumptions, of how it applies, of what might need to be looked at more closely. Of course, there are some terrible ideas out there. But one must first see why they are held on to, regardless of their flaws, and then find ways to improve the idea. Question to understand, not to disprove. Help the person and be willing to find your own ideas challenged and amended. Don’t take the easy way out by only poking holes in a theory; be responsible and improve that theory.