I'm not speaking for Blizzard, they've already spoken for themselves:
Blizzard has stated that the reason they only have one DPS spec is because they couldn't think of enough unique mechanics for a second spec, so they rolled all of the ideas into one. The only thing unique to Havoc right now is repositioning and skillshots as part of the main rotation. Thus, if you read what Blizzard has already stated about why Havoc is the only DPS spec (hint: it's because they only want to add specs now if they provide something wholly unique), and you look at the one unique thing that Havoc provides over other DPS (hint: it's repositioning and skillshots aka Fel Rush) it doesn't take a whole lot to come to the logical conclusion they Blizzard fully intends Havoc to be centered around their completely unique mechanic of repositioning as part of their core gameplay.
This is further evidenced by the fact that Fel Rush has existed for at least 9 months publicly, for over a year at least internally, and was featured as core gameplay in the initial class reveal as Gamescon 2015.
Take this mountain of evidence, and combine it with the fact that builds that ignore Fel Rush / Fel Mastery are up to 20% behind builds that include it (hint: Blizzard is indirectly telling you how to play the class properly here), and the
only conclusion you can draw from this is that Fel Rush is completely and assuredly the reason that Havoc exists. The entire spec is built around this mechanic of a repositioning skillshot.