PostgisDialect extends PostgreSQL82Dialect with Postgis specific datatypes but PostgreSQL82Dialect has been extended with PostgreSQL9Dialect which is extended by PostgreSQL92Dialect extended by PostgreSQL94Dialect.
These PostgreSQL9*Dialect classes adds features from newer Postgresql versions.
At the moment one have to choose between using PostgisDialect to get Postgis features and a PostgreSQL9*Dialect to get access to the latest Postgresql features.
I suggest to create a PostgisDialect per PostgreSQL9*Dialect to get both sets of added features.