Look at tp FAQ at tpnimi.blogspot.com for more details. Basically, without those breaks (and we need one for prepositional phrases, too) we would just have a string of undifferentiated words with no visible structure, since tp has no rigid word classes. We could take any substring as subject, the next as verb and one more as object, any which way.
I admit that 'ona' is a tricky case, but the thinking seemed to be that it was more likely than 'mi' and 'sina' to get modifiers and also that it was far less common. The latter has turned out true so far, the former less so.