Introducing new types converting dates into a DateTimeImmutable rather than a DateTime could be useful for applications prefering the use of immutable datetimes (and relying on PHP 5.5+).
The existing types already support setting a DateTimeImmutable in a field mapped with the datetime type, as it does not check the value against DateTime but only expects a format method. but the conversion from DB to PHP will always produce a mutable DateTime instance, thus preventing to use the immutable flavour consistently.
Not that this is a low priority issue as any code interacting with third party packages will probably need to use DateTime anyway because of the typehints (typehinting DateTimeInterface would not work if you need to keep compatibility with 5.4)
If DBAL 3.0 bumps the minimal supported version to 5.5 (which might be possible depending of the release date of 3.0), we could decide to break BC and use the immutable flavour in the datetime type directly.