I'm confused and am not sure if you're thinking of something else or if I'm quite misinformed about Lojban. I don't think lojban has suffixes at all (prefixes, on the other hand). For -j specifically, the only words in lojban which are permitted to end in j are names, but there it's not obligatory. Names are required to end in a consonant, but there are at least a few that are more common than j, I think.