The purpose of the lnb cover is to protect it from getting moisture inside. I have seen small cracks lead to clinging moisture on the inside of the cover...This has caused some signal loss issues and I have replaced the cracked cover when I run across them....I live in a place that gets a lot of driving-sideways rain and clinging snow....So, if you live in a dry place, I don't think that I would worry about it all that much...But, if it was listed as new, then get a replacement.
Good Luck!