After all the world tour final is the best players of the season playing to show who is the best of that season. You have to do very well in all the seasons major tournaments to even get into the WTF. This year it currently takes 6000 points to qualify. To reach that many points over a season you have to be a very good player.
In the world tour finals you only play multiple matches against the top players on hardcourt (which is the the most common surface in the ATP and therefore should be the most prestigious) and indoors where there is no wind/rain or other factors to benefit less skilled players or cause interruptions, delays that can throw off players serve or offensive game.
In short the tour final should be worth MORE than a slam. Currently a win only gives 1500p. This is basically for historical reasons. The WTF has never carried as much prestige because it took a while before they found out that tennis should also have a "playoff" like many other big sports.
WTF is basically the tennis playoffs. The best players facing each other to show who was the seasons best player. In many sports the regular season is even seen as pointless if you don't qualify for the playoffs and do well there. I tend to view sports the same. A win in the faceoff between the very best is always very valuable! If it's not as valuable as it should be compared to regular season there is a problem.
So should WTF be worth 3000 points with SF being worth 1500? Should they change it and have the 32 or 16 best players of the season fight for victory instead. Should the final be best of 5? Or even best of 3 but played in a best of 3 matches? I think its a disgrace that the "playoff victory" isn't even worth as much as a slam when there are 4 of those in a season! There is only 1 WTF in the season.
I'm not sure it should be worth more, but it should be equivalent.
Everyone will jump on you about matches not being contested over 5 sets. Watch and see.