When I first started as a host I went to and registered on just as many forums as steve did. If there were limits of less than 10 a day I probably wouldn't have stayed because I wanted to break the average 10 post count. I don't spam mind you but it would have made me forget about HD. By far HD is my favorite as art is patient and gives you chances while other forums simply give you infractions (wht) or ban you. But, I wouldn't have found this out if I couldn't make the initial ten post. When I first started I was hungry and wanted to post on as many forums as I could and a forum that limited me would have been put in last place while those that didn't would have been my primary focus.
If there is potential for a post limit, let the limit be something high but not too high. Maybe 8 post? This way the person can't spam his way to 10 and has to wait for one more day. Doesn't ruin the experience and makes it harder to post an ad if they do decide to spam. Gives art time to catch them on day one before they post spam all over the forum just to post the ad. By day two, the spam would have been removed and the person would have been warned without ever having his ad shown.
Not sure if my ramblings made any sense