As you've alluded to, it depends on how 'necessary' the part is, and whether or not it makes a major aesthetic/functional difference to the toy. To use your example, if I had a G1 Blurr, I would definitely want the shield.

When I had G1 Snarl, I didn't care that I only had one out of three missiles, since I'd only be displaying him with one missile anyway.

I thought my G1 Hound was complete, but it wasn't until a while later that I realised the little jerry can that attaches to the back of the jeep was missing. This didn't bother me that much, either, since he won't have it on in robot mode, and in vehicle mode I'd be displaying him front-on - also, Hound's jerry can is surprisingly expensive. Something like his gun or missile launcher, however, would be more noticeable, so I would shell out for that (as I did with my G2 Slag's gun).