Logstash used to support --grok-patterns-path, but support was later removed. The rationale was that grok plugin was the only one using the flag support. However, the only reason --grok-patterns-path was added in the first place was because the configuration language does not support any "global" options at all.
I have the same problem with "patterns_dir", but also with many other options. For example, I use "start_position => beginning" in all my file inputs, and many other options which lead to needless duplication inside the configuration file.
Please allow specifying default values per plugin in the configuration file (perhaps input_defaults block, etc.?) or allow merging or inheritance in the actual configuration blocks similar to Yaml merge: http://yaml.org/type/merge.html