It's pretty well accepted that the wild 8 is more weight than the last 2, and that the call 8 is closer to the last 2. Logic will tell you the same. That is, with the wild 8, you can potentially win with 2 balls (8 or 9) on any shot. With the last 2, you can only win when there are 2 balls left. AND, if one of them is the 8, you would win anyway. So, the last 2 could ONLY better a better spot when the 8 is made early by your opponent. Think about how many times your opponent has made the 8 by combo or carom early in a game. Very few. Simple math will tell you that the wild 8 will be better over the long haul.
A simple thought experiment will illustrate that the last X is never as good as the corresponding wild ball(s). If the last 2 was better than getting the 8, then the last 3 would be stronger than getting the 7 and the 8. And the last 4 would be stronger than getting the 6, 7, 8. In this last example, you have 4 chances to win every shot with the wild balls. But you could only win late in the game with the last 4. Consider if your opponent missed the 4 ball and left you a combo on the 4-6, 4-7, or 4-8. You would win with the wild balls, but not the last 4. But, what if your opponent played a mediocre safety on the 4, but the 6, 7 or 8 was hanging somewhere? You still couldn't win with the last 4 without running balls, but you COULD win with the wild 6, 7, 8. There are many other scenarios to contemplate, but virtually all of them result in the wild balls being a better spot.
Lastly, if you ask anyone that's gambled a lot, they'll confirm that they would rather give up the last X, or a call ball, than one or more wild balls. If someone tells you different, that probably means that their skill set is mediocre, they gamble with ATMs, or they haven't really gambled enough for statistics to prove itself.
-td
PS - here's a thread from a decade ago on this. The last 2 is almost the weakest spot you can get. Times haven't changed...
http://forums.azbilliards.com/showthread.php?t=103835&highlight=spot [Note, "the breaks" can improve the spot marginally.]