Both have pros and cons and it's not the end of the world to have one or another.
The explanation of the advantages of the thanks system does make sense indeed. But I was thinking, would some like to have both otherwise? I've seen this on several forums.
Post count can indeed lead to spamming but as Riku says, it also is some kind of indication for new people if the join date is added. I know that it can be intimidating but, well, that's how it is in life. Being new somewhere can be a bit of a challenge ;) On the other hand, I think negative remarks towards new people probably do more to scare them away than a friendly regular with a high post count. With both systems, they could even make more of a distinction when comparing, post count, join date and thanks.