Lets see... what would I do if I were a "smart Shinzon"?
a) I know I will die in minutes without Picard's body.
b)The Enterprise is without any weapons. She cannot hurt me.
c)My own ship is virtually unscathed and heavily armored.
So what should I do? According to the above, I should:
a) Stay out of transporter range of Enterprise, preventing me from abducting Picard.
b) Stay cloaked, preventing me from abducting Picard.
c)Keep my shileds up, preventing me from abducting Picard.
Add to this the facts that:
a) Shields have NEVER prevented large, massive, solid objects from penetrating them and striking the ship's hull.
b) The Enterprise is a large, massive, solid object.
c) I need Picard's body in order to continue living more than a couple minutes more.
d) Even if Enterprise ramms me, my ship will only get a little scratched, and Enterprise will be devastated.
You didn't answer my other questions. Did Shinzon try to shoot out the Enterprises impluse engines when she started her run? Nope. Did he try to lock on a tractor beam? Nope. Considering that the Enterprises shields where all but gone, did he try to beam out Picard and simply destory the ship? Nope.
And didn't the Scimitar transport over troops to the Enterprise while she was still cloaked? Or was this after they lost there cloaked?
Also the Enterprise shields stop a large, massive, metal object (a Valdores blown-off wing). Considering how much bigger and more massive the Scimitar is over the Enterprise, having its shields up would of greatly reduced the impact, it not stopped it completely. Even if he kept he shields down so he could transport over Picard, why didn't raise them when Enterprise started her ramming run? She had plenty of time.
On a closing comment, the Scimitar was more than a little scratched after the ramming.