I usually just clean and return. I've been debating adding some type of fabric protectant as an add on, but they are usually on the more expensive side so I wouldn't just include it for free
IMO the only potential benefit from ScotchGuard/etc. is that such stuff *might* prevent stains from setting as badly as they might without it.
I can't imagine it actually providing any physical protection against the sort of "dirt grinding into the fibers" damage that happens when dirt gets on (and thus into) the carpet. Like...it won't prevent the "on = into" to an appreciable degree.