bots can slow down the sim cause they are avatars just the same. However what happens is people maximize the effectiveness of thier bots to be low impact but cause high traffic.
I have literally watched a new hangout go from bottom of screen to top in matter of 3 days. On the first day I was shocked and looked to see what happened cause the place had been around for a few weeks. I found 5 people with the exact same name <stupidname>bob, <stupidname>jane, <stupidname>john, <stupidname>charlie, <stupidname>tom. All born on the same day, only three groups visible almost all related to the sim, no picks other then those to the sim, etc. A few others I noticed are always there but they had varied picks, filled out profiles, tons of various groups, and were kinda hard to tell if they were bots or not (likely not).
a few bots might not seem a big deal but once they reach a certain level in search they can rise quickly. specially in areas were they cater mostly to say a popular group (goreans, vampires, goths, etc) but what you are searching for a sub group they tossed in to get more hits in search.