The term 'short read' came about to describe technologies that produced reads that were substantially shorter (30-50bp) than the mainstream technologies employed at that point (1000bp). The 'short read' sequences are getting longer as the technologies evolve but as far as I know there is no length at which sequencing would be called 'long read'.
Short read technologies produce higher coverage. Longer reads are easier to process computationally and interpret analytically. A trade-off between the various needs and requirements determines the right choice.
As mentioned, I believe that there is generally a trade off between length of sequence and number of reads (coverage).
Sometimes longer reads are needed. For example, to cover more than one exon/exon junction in mRNA to deduce isoforms.
Also, the way a library is prepared is usually very important for analysis. Often times an RNA sequencing experiment will remove small RNAs from a library before sequencing - microRNAs, siRNAs, and other small RNAs will not present in the sequencing results, so these libraries can't be used for projects analysing those.
Just to add a little:
As mentioned, I believe that there is generally a trade off between length of sequence and number of reads (coverage).
Sometimes longer reads are needed. For example, to cover more than one exon/exon junction in mRNA to deduce isoforms.
Also, the way a library is prepared is usually very important for analysis. Often times an RNA sequencing experiment will remove small RNAs from a library before sequencing - microRNAs, siRNAs, and other small RNAs will not present in the sequencing results, so these libraries can't be used for projects analysing those.