It would be possible and very flexible, and certainly
more powerful than the
current system. But we would loose the convenience of having one date, which
we need for query answering (or we could default to the lower or upper
bound, or the middle, but all of these are a bit arbitrary).
I believe it would be more profitable to build a query system which
always queries for the range. This would work for interval-only values
(see my comment on the wiki page) as well as for value with interval.
I don't see this as a big overhead. It is more a problem for ordering,
but internally, wikidata could store a "midpoint" value for intervals
where no explicit central value is given, and use these for ordering
purposes.
I think it would be great if the system is consistent for quantities,
dates, geographical longitude/latitude, etc.
Gregor