Despite all the criticism against Esperanto's letters with hats, there 
are in fact some good reasons to use diacritical symbols in auxiliary 

1. Adding diacritical marks to existing symbols is a typical way to 
extend alphabet. Diacritical marks are used in most modern writing 
systems including Latin, Greek, Cyrillic, Arabic, Devanagari, Katakana 
and even Bopomofo.

2. Diacritical symbols are distinctive in a good way. Jespersen left the 
letter C out from the original version of Novial because he concluded 
that it was too chaotic. It is used for representing too many different 
sounds in European languages, including /k, s, ts, tʃ, θ/. A diacritical 
version of C, such as Ĉ, is not the same thing. It clearly signals that 
the symbol represents a different sound than C in your native language.

3. With diacritics in your repertoire, you can avoid using plain letters 
in unexpected ways. I use <x> to represent /ʃ/ in Pandunia and it looks 
strange. But there aren't any better alternatives left in Basic Latin, 
so... :(

4. Diacritical symbols can help to retain familiar appearance of words 
sometimes. That's why Esperanto has hard G and soft Ĝ.