Yeah, I know what you mean. This is one of those figures to me where it'd be pointless to have both. Other than the shield and the base, I don't really that much of a vast difference. I already have the regular version on PO and will by all means stick with that one. If I do get anything from the promo figure, it would just be the shield. However, as you mentioned, that probably should have already been an accessory with the regular CW Cap figure.
For those that passed on the regular version though and went for the promo one, I don't see anything wrong there. Either one is going to be a great figure to have. But I see utterly no reason to have both, in my opinion.
And this is coming from a staunch Cap fan who has every Cap figure thus far.
The best CW Cap figure to me is going to be an amalgamation of both figures; regular version with the BD shield, or the battling version with the sculpt that has the interchangeable face pieces.