For example, in Russian pronunciation, you might definitely confuse 'ti' for 'si'. If Russian wouldn't have the real 's' consonant (or 'ц'), it would make sense to transliterate 's' as 'ti'.
But english is (more or less) a phonetic language. If you're romanizing a foreign language, and it doesn't come out more or less phonetic, then.... try again. It doesn't cost anything to keep an "s" in the prospective alphabet.
the "s sound" (IPA [s]) is essentially an allophone of their "t sound" (IPA [t]) before 'i'. Since they don't perceive them to be distinct phonemes, there's no need for a separate letter.
And 'z' sound in Italian means two different consonants; in English it also means two different consonants, both also different from both Italian ones. Both are "phonetic".
A not entirely irrelevant fact about "Kiribati" is that is the same word as Gilberts -- as in Gilberts Islands.