Traffic numbers, regardless if it using reader mode or not, are used as a basic valuation of a website or page. This is why Alexa rankings have historically been so important.

If Perplexity visit the site once and cache some info to give to multiple users, that is stealing traffic numbers for ad value, but also taking away the ability from the site owner to get realistic ideas of how many people are using the information on their site.

Additionally, this is AI we are talking about. Whos to say that the genrated summary of information is actually correct? The only way to confirm that, or to get the correct information in the first place, is to read the original site yourself.