Cliff notes- Nobody likes paying for DLC, but PD is pretty fair about it compared to other companies. It is also a good way financially for a company to continue paying employees to patch/support a game
As much as I hate paying for DLC as much as anyone it's unfortunately the way the gaming market works now. Some games came out using the micro transaction model, and where extremely successful and profitable. As much as I'm not a huge fan of this because I'm a consumer it works for the companies.
Also the fact that it does work for the companies also gives them a reason (and now with online gaming a means of distributing) additional content. Do I think that some games have things held back from the release to later be sold as "add-on" content? I'm sure some of them do this. PD is not a main violator of this though in my opinion. Also their pricing seems generally fair enough to me (though with the exchange rate I can see someone outside of the US being upset since their /£/etc is worth more than my $). I don't however think most of the things they've released were actually held back from the release to later be sold, I could imagine that Spa, Route X, cars, etc may have been being worked on before release but I don't know that these things were actually completed by release.
My only other issue with this is the fact that for quite some time everyone was complaining and wishing they'd just release the game. They finally caved and did this (probably with a push from Sony. I'm not sure if it's admitted or not but on release everyone knew the game wasn't 100%), release the game, continue with terrific support in the form of free patches and some free cars, and we then complain about them charging for some DLC.
As if they came up with this on their own. Tigerwoods 2012 comes out to be around $110 for the total package (last time I checked), and that additional money comes from purchasing courses that are part of the season, so if you don't buy the courses you can't actually complete a season. Where as PD say's "here are some tracks and cars but you aren't required to have them to get through A/B Spec, they're just for practice/online/seasonal. Oh and for a couple months you can play the tracks in someone else's online room if they have them... and after that we'll throw them in some seasonals so you can still use them in a limited manner." Again, it isn't my favorite way to do things but they are being as fair about it as I can imagine a company being.
I do have all of the DLC to this point, and chances are good that I'll buy all the packs that come out in the future. I don't like paying for it (we would all love more content for the same or less cost) but paying for it doesn't only support them bringing out more DLC, it also supports future free patches and updates which again they've been terrific about (especially considering that it isn't a subscription based game).
Sorry for the long post.