For this discussion, we are talking abotu the value of a goal or assist in regards to evaluating players, not impact on the games.
At best, a secondary assist can only ever be equal in value to a goal. It can never be higher. Regardless of how amazing a secondary assist may be, it does not make on to the scoreboard without the goal scorer. Does not matter if it is a tap in, and deflection, a rebound, etc. If the goal scorer does not put the "shot" on net and past the goalie, that second assist does not exist. The goal scorer had to get in the right place, at the right time, and do the right thing (even if it means sticking his ass out for the puck to bounce off of and into the net).
The only goals that should be considered less valuable in this sense are the own-goal variety, where an opponent scores on his own net and the goal is awarded to the last "attacker" to touch the puck (or the center that lost the faceoff).
On the flip side, a second assist can scale down in relative importance to the play as compared to the goal. It can be almost meaningless (but never totally meaningless, else there would not be a second assist).
All that being said, in evaluating players, using simple raw numbers such as G, 1A, 2A, etc. is highly suspect. In comparing players, if there is a significant difference in scoring rate, the player that is involved in more goals will usually be considered the better player. If the difference in PPG is negligible, then we should value goal scoring over assists, as explained above. A good example of this is the 2003 Ross/Hart/Pearson results. Forsberg led the league in scoring (29-77-106 +52), his childhood friend Naslund was second (48-56-104 +6). Forsberg got the Ross and the Hart, Naslund got the Pearson. It is a worthy debate for either player. As it was, the writers thought Forsberg was more valuable to his team (2-way play drove this in addition to point totals), and the players thought Naslund was the better player (pretty sure more goals prompted this). Personally, I am a Forsberg homer, so obviously I was happy with the result. But I would not have argued much had Naslund gotten the Hart.