Why the long-short romanization system (长短音) for Chinese dialects?
This is a bit off topic from the category of Chinese, but most languages that have never been written before (i.e. they were spoken languages with no writing systems) do not choose the IPA or even the most common writing system used in their area (often seen as the alphabet/language of the colonizer, which may be the sentiment reflected in China on a low-level with the Mandarin-speaking Han representing the colonizers) to transcribe their language for the first time. Often, they use something distinct in order for readers to know in which language it is to be pronounced (in the case of Chinese) and to proudly display their distinct heritage. 

If you want more information about this, there are many linguistics papers on the topic of the choice of alphabet for Haitian Creole, which is currently being debated at the national level in Haiti. Long story short, the choice of alphabet has more to do with heritage and aesthetics than logic and science... those dictionaries are using 长短音 to differentiate themselves from ones for more established dialects.