As one who campaigned hard for the change in the system that only allows those who buy cards to rate them, I still think that it is a lot better than the previous system, although it is probably the main reason why most new cards are higher rated than older ones.
And yes - there is some injustice in that many truly fantastic older cards are rated far too low, although I do think the general quality of both footage and performance has improved over the past couple of years. However, I still think the current system is much better than the old one, where any individual could set up as many profiles as they liked and, therefore, rate a card as many times as they liked - leading to "ratings attacks" on specific girls or "ratings hikes" for others - which massively skewed the ratings, making them much more unfair than they are now.
Now under the current system, there are 4 main rating issues as I see it : -
1. Many of those who rate cards are already a fan of the specific girl who's card they are buying and are likely to rate it as 10 anyway, regardless of content.
2. The number of ratings for older cards is usually greater, in many cases by a huge margin (A card released in 2007-2008 will have a lot more owners, and therefore ratings, than a card released in 2010-2011). So average rating tends to be lower for older cards due to sheer weight of numbers.
3. For older cards it is likely that a higher proportion of it's owners are subscribers, who get cards at random and may not necessarily like what they see so much, so they are likely to give lower ratings.
I've noticed something similar when VIP cards are first released. Because VIPs get them automatically for free, and would not necessarily buy them, the initial overall ratings tend to be lower than for non-VIP cards until the effect of buyer ratings kick in some time later.
4. I think some (or perhaps even many) people are still just not very discerning and tend to rate cards based on how much they like the look of the girl, regardless of content or performance (something I always try to avoid when rating and reviewing - although I admit I've been guilty of that in a very few cases).
As I said though, I think the current situation is far better than the old days of ***** and ratings wars instigated by multiple profilers, many of whom didn't even own the cards in question.
The ratings system is just a rough indicator of popularity, and is inaccurate at best and misleading at worst in terms of quality. That is why I always recommend reading the more detailed reviews before you decide to buy a card, rather than just making a decision based on a rating which very likely gives little idea of the actual quality or performance.