You are right that one can imagine a dialect with not more than two slots.
The advantage is that there are fewer slots to remember.
The drawback is that there are more words to remember.
You can create such a dialect within Lojban and it won't break Lojban. You are even free to add words for it to
jbvovlaste.lojban.org.
Whether they become popular is another question (you would probably have to write texts in Lojban so that others read them and absorb your new words).
The same applies to three-slot predicates.