If this site was fully funded by the owner, I would vote yes but considering part of the costs to operate this site comes from paying sponsors, I just do not think it would be fair to allow commercial posts when they do not pay for anything. It really becomes free advertising for them. You've tried it in the past and it didn't work, and most likely history will repeat itself. $$$ corrupts people, even the ones we come to know and trust.
These non-paying companies can host their own forum/site to answer any questions people have. The little product knowledge we would gain from it would not be enough to make up for the product pushing we will eventually see if we allow something like this.
Allowing commercial posts won't stop spammers and shills to come here. This is probably the most visited auto-detailing website. If you're here often enough you begin to get an idea of who the possible shills may be. Some of them are well trusted by the community, but it's up to the individual to make up their mind and decide whether or not they should take that person's opinions with a grain of salt. When it becomes really blatant though (like Justin, even though he was never technically banned for shilling), that really bothers me. I can put up with the other ones.