Probably as some sort of eventual irony, but then, for Rand, Verin was the only Aes Sedai he was familiar with, and had travelled with, save Moiraine. Alviarin was someone rather unknown to Rand, and with all that sucking up in her letter, mistrusted. Moiraine just chose an Aes Sedai Rand knew to make the comparison. It wouldn't have done to say "Trust Nacelle as much as you trust Alviarin." simply because Rand doesn't know who Nacelle is and isn't familiar with her.