I don't like diacritics because they're generally ignored in informal writing. Have you seen chatting Vietnamese or Czech? Most of them write text just like they would write it if there were no diacritical marks at all. Of course it doesn't really bother anyone, but it's still no good when every third word has an incomplete appearance and you shall decipher it (the text is uncomfortable to read).
Different glyphs for different sounds make them also more "stable": for example most of Russian speakers don't even realize that the letter "ц" (ts) is just a compilation of two sounds "t" and "s", and is not an independent sound.