Although to some it may appear as a nice simplification, I shall strongly discorage using optional keywords that do not add to the semantical meaning of the expression.

For a language it is important to be clearly understandable and easily learnable, and readable. Therefore, I would strongly suggest either making the keyword "units" non-optional but explicit or alternatively not using it at all. Please do not artificially increase the language complexity.

Moreover, as it is now, the syntax does not seem to be uniform any more. E.g. compare the two rules:
* valueQuantity\.value = 55\.0
and
* valueQuantity units = UCUM#mm "millimeters"
I would much rather like having
* valueQuantity\.units = UCUM#mm "millimeters"
instead, as for me units can be seen as an attribute\. On that note, the binding to externally defined units:
_* valueQuantity units from http://hl7\.org/fhir/ValueSet/distance-units_
would be better uniformly re-written as
_* valueQuantity\.units = http://hl7\.org/fhir/ValueSet/distance-units_

Existing Wording:

Note: Use of the word units is suggested for clarity, but is optional.