While quarantining is something that should be done, I will admit that three out of the five rats that I have were never quarantined. They were emergencies and I never had the time to prepare for quarantine. Two out of the five I have now were quarantined though. One of them, Toast, I bought when he was 10 days old and he had his own seperate enclosed area while I raised him. He had 24 hours of attention 7 days a week unless I was at work, eating dinner, going to class, or doing homework. By the time he was old enough to join the others he was quarenteened. My oldest rat, Pastoolio, was also quarenteened at my boyfriend's house two weeks before he came to my house. He also had constant attention and snuggles. Once he was at my house he spent the next few weeks in his own seperate cage because I had him neutered.
While not everyone does quarantine all the time I do recommend it. I've had to quarantine a friend's rats before because they had mites. Basically a good reason to quarantine is because you don't want bugs or illnesses to affect other pets or other members of your household. Let's say I didn't quarantine my friend's rats. My dog and my other three rats would probably have mites. Once I treated my pets for mites I'd have to treat the carpet, bed sheets, and clothes for mites as well. Quarantining my friend's rats saved me time, money, and possibly veternary expenses.