UTF-8 is a nice format for storage/transmission. If you're going to do some heavy processing with your text you're supposed to convert it to a fixed-width format in memory (typically UTF-32).
Most heavy text-processing applications I know actually tokenize text to words (well, technically terms), keep a lexicon mapping from the term ID to textual representation, and then work in term space. Individual letters are usually not semantically meaningful in most languages (both human and machine), and so your analysis becomes much easier if you operate in a space that is semantically meaningful.
If you do some heavy processing with your text, the triviality of decoding UTF-8 (or anything else) to codepoints is no issue compared to the complexity of actually processing text. If you think UTF-32 makes it (significantly) easier to do text processing, you're not processing text you're destroying it.
