The current use of UNIX timestamp is a compromise, and has its issues, but still allows in-SQL operations.
In-SQL operations is very very important. Going to ActiveRecord type of abstraction without the ability to do aggregate operations on the data is just not an option.
Without in-SQL, doing things like:
select n.nid, n.type, n.title, count(*) as num_votes, avg(value) as avg_vote from votingapi_vote v inner join node n on content_id = n.nid
where
n.type = 'image' and n.created between unix_timestamp('2007-11-01 00:00:00') and unix_timestamp('2007-11-30 23:59:59') group by v.content_id having num_votes > 1 order by avg_vote desc, num_votes desc limit 20;
Would be impossible, or would entail very expensive full table scans.