I agree, grasping the hands is unnecessary for this pose. If you have a shoulder injury, do what feels right! I have no problems with a straight arm plank, it will just place more load through the upper body...but that's definitely not a bad thing if you're looking to rehab the shoulder.

I'd also recommend taking a look at exercises like wall sliders, wall angels, plank shoulder taps, and kettlebell Turkish getups. These are some dynamic exercises that really helped in my recovery. Please take them with a grain of salt, however, as I don't know your specific situation.

Cheers! :)