I am currious, if you do not have shield block and you are wielding a shield, is it the same as if you were not wielding a shield?

Firstly, if you have armor use skills (e.g. light armor use), the shield is about the most important bit of that armor. So it will help you slightly there in stopped blows hitting you.

Secondly, even someone that doesn't know parry, or shield block, or dodge, can do it now and then. Unless, of course, they are not holding a shield, in the case of shield block.

Thirdly, you may gain some effects by not using a shield (e.g. backfist), but you lose the benefits of the shield (e.g. extra hp).