I think these characters shouldn't he allowed in titles. They make it difficult to spot serious topics. But forum cannot ban unicode because the languages sub forums needs them.
There are ascii tables for each language which (with exceptions) represent most well. But yeah, some specific language boards could face problems (probably the Chinese ones mostly) although I'm no expert on the topic. English however, which is the only language used in all major boards is the best-represented language by ascii given that English is the language for which ASCII was invented.
I remove or limit them sometimes when they are either excessively annoying or distracting, they definitely shouldn't be interfering with other topic subjects.
If you don't mind picking such topics one by one on your own then I guess it's not a problem. By now, this is probably not abused to a large extent. If it ever becomes a problem, a non-invasive way to prevent such abuse would be to make a list of said characters.