Most people ask for RP samples so they know what to expect in rp. For example, if everyone around you is using perfect grammar, punctuation, capitalization, spelling, etc, it's better to be judged by one person rather than everyone around you. Though unlike a lot of people, when I require and ask for rp samples, I'll usually accept people who don't have that 100% perfect grammar that all the other elitist literate rps require. Which I agree, those types of people are very.. ah, well, not good. Though it is fair to ask for a role play sample if you want everyone to be on the same level and same page as everyone else. For example, if someone writes perfectly, no spelling, punctuation or capitalization errors and makes like 3-4 paragraphs per post, I think if you were to accept someone that writes a half to 1 paragraph and who has multiple spelling/capitalization/punctuation errors, they'd feel a little intimidated by those around them.
That's how I look at it, anyways. I'm always willing to accept people who make a few mistakes in their samples so I can help them improve overtime while they're in my role plays.
I hope all of this makes sense to other people and I didn't just confuse them with my rambling. xD