I voted to have everything unlocked in Arcade from the start. I think someone else in this thread already mentioned it, but I don't see the harm in doing this, as long as you're not able to modify them in Arcade at all. Sure you should be able to choose tire, and driver settings...but zero tuning, visual mods, etc for arcade.
Therefore, if you REALLY want the car and want to be able to tune it to your desire, you would still have to unlock it to do any modifying. It would still give you a reason to do so if you love unlocking things. That way, everyone would be happy the way I see it.
For me, I feel like I've been collecting cars forever in GT5, and I think I'm still well under halfway to 1,000...I don't have a ton of time to play it anymore, but I love driving the cars in general more than anything. I hot-lap quite often for the heck of it, and collect cars, but rarely drive some of them because there are so many to buy and use..but at the same time, if something rare comes across in the used car lot, I can't say no to it because who knows when it's going to show back up. So it tends to take up majority of my time in GT with how I play it at this point.
I would love to be able to just turn on the game sometimes and be like...hmm...I'd like to drive a Saleen S7, an Aston Martin DB7, and a 1960's race car. Just trying to find some of those types of cars in the game sometimes is more of a chore than a joy. I will get my racing fix with the cars I want to drive at my finger-tips....and then if I want to tinker with them, then the chase begins in GT mode. I'd be a happy camper.