I'd be surprised if this ends up being exploited - from a practical perspective it'd be extremely high effort for comparatively little reward. Both platforms are ASLR now + no bundled scripting ability in tiff + manual work flow (ie you can't feed it to an automated process 10,000 times in a row) + low install rate (compared to say, flash or qt).
Hell, at least Adobe doesn't charge you for a support contract just to let you read the CVE with WONTFIX :D
This vulnerability hasn't been seen in the wild they say. And yes, the likelihood of someone opening a TIFF file they received by email in Photoshop is low. A lot of work to create this virus for very little payoff. The people who open email attachments are not the same people that have Photoshop installed. And in this case, you'd probably need to save it to disk and specifically open in Photoshop since most people won't have TIFF associated with PS automatically.
It's a tough call to fix for every possible version of Photoshop affected when the likelihood of it even happening is very very low.
Hell, at least Adobe doesn't charge you for a support contract just to let you read the CVE with WONTFIX :D