Some people live in areas with bad internet, others might have households with 3 or 4 people all trying to use the WiFi at the same time. Whatever the cause may be there are plenty of genuine reasons for why there might be a lag spike so nobody should ever jump to the conclusion that their opponent is cheating. Of course lag switching does happen but anyone accusing someone of lag switching when they have zero evidence is just a salty scrub who takes online gaming far too seriously.
Then there's separate situations where you can:
1) Prove with 100% certainty that a user is lag switching, such as in a case where you forensically test for it with a toolkit named "punkbuster" (appropriately named, btw).
en.wikipedia.org
2) And/or in situations where a user brags about doing so in writing.
3) A person who endorses another fraudulent player are the folks you have to watch out for; not the person calling out frauds...
4) Online play can also be just as competitive (at a tournament level) as offline play, although not with NRS titles, since they have a reputation for shady net code. Gears of War 3 or 4, for example, had dedicated servers and you can play tournaments online at a high level...
5) ALSO: If the entire online portion of a video games competitive sector shouldn't be taken seriously, then why would people even pay for it or take the competitive aspect of the title seriously at all?