HoRNet Plugins has released an update to AutoGain Pro, the gain automation utility.
The update is a bugfix and includes:

• Improved smoothness of recorded automation.
• Fixed automation writing jumps in Logic X.

The update is free for current owners of the plugin and can be downloaded from the user area on http://www.hornetplugins.com

I got Mk2 recently, so I'm wondering if there's any reason to still use Mk1.

I was wondering the same thing. I went ahead and installed the update. Reason being that while both MK1 and MK2 can be set to "classic" mode (the original AutoGain, still available), MK1 seems simpler to use. So, unless the additional detectors, etc., in MK2 are needed, might as well use MK1.

Fleer
I got Mk2 recently, so I'm wondering if there's any reason to still use Mk1.

Probably no reason except to retain compatibility on previous projects that used Mk1.
 
EDIT:
Another reason to keep mk1 would be because the interface is simple and possibly easier to use for certain (quick) applications.
Use the mk2 when you need to bring out the "big guns" for a more sophisticated or difficult mixing situation..