It looks like you can get a wrist strap for the Motoactv. This might be an advantage over using your phone.
I still use a Polar heart rate monitor while I run even though I'm tracking my run on my Android phone. This is mostly because it's more convenient to see the display on the wrist watch unit, than to have to fool the display on my phone.
This might not make it a big enough advantage if you already have an Android phone, but if you were considering getting a Garmin or Polar device for your exercise, the Motoactv might look pretty attractive.
- John