After owning the game since last Friday I can honestly say I'm not bothered. If the games economy was so broken I hadn't been able to afford any of the cars I want or need so far then it would seem like a dirty move, but there's not shortgage of ways to earn credits from actually drivingand racing whith or without the mod cards. I usually forget to enable the mod cars in most races, but even then I'm getting decent credit payouts for 7-9 lap races in the career and in free play. The only loot crates I've bought we're to see if I could get lucky with a legendary car, but I hand more than enough credits to buy it and not be too bummed I didn't get one.
I don't want to be able to afford a supercar after 10 laps, but I don't want attempting to obtain one take 20 days of grinding. Which leads me to another issue, if you'reobsessed with only beating the career modes as fast as you can, you "may" I imaginehave to re-do a series or two twice to buy the cars needed, I've not had to so far, but I've been splicing my time in Free Play driving the cars I've won or bought when leveling up as well as the ones I've actually spent my credits on.
The system to me seems very balanced at the moment and while sure, it is a cheap way for Turn10 to make more money out of people, those that use it are doing so willingly. I'm not a fan of the practice, but it certainly doesn't ruin the game for me to know they are coming, assuming they don't change the current payouts (excluding any known exploits which they have every right to fix).
Besides as
@SlipZtrEm said, for some people time is money. I don't spend a tenth of the timeplaying games now than I did 6 years ago and while I'm never persuaded to pay for faster progress in games, I don't judge those that do. Whose to say why some people send £10's of thousands on online games (which some people do), it's up to them if they have the money and enjoy it. People saying it's like paying for nothing, for free progress, isn't that what buying a game is anyway, we're buying something that allows us to make fake money to buy fake cars we don't own ato drive onfake tracks in a virtual envirnment. We do it because it's fun, some people might have more fun if they progress faster, it's up to them.
In conclusion, as long as it's not forced, and the economy isn't changed to tie in with these MT's, I have no concern. Should they change the economy to make earning £100k a full days work then Turn 10 would be wrong IMO but that's an if that I don't beleive have been supported by previous titles, so let's wait and see.