Yes, if you have the Apple ID and password then you could view all iMessages on another device. I think it sends a notification that their account was activated on a new device. I would only do it if you could intercept that message.

For example, I can activate my phone number on my iPad Message app. I only get iMessages on it though, not SMS text messages.

BW (me)-31
WH - 33
M - 5 years
DD 3 years old
EA/PA - 5/2013
D-day - 6/29/2013. He refused NC.
I moved out with DD 9/2013.
Sold our home and made divorce arrangements.
Plan B - 12/8/2013
FR - 1/2014-3/2014
Divorced 4/2014