There are obvious problems using double-dummy data to determine valuations. The first is that it undervalues queens. Double-dummy, I can always make 3 tricks in the suit:
KTx
AJx
That's because, double-dummy, I always know where the queen is.
Similarly, when I hold:
Kx
AJ9
Double dummy, I can make 3 tricks whenever east has the queen or the ten - about 3/4 of the time. (If east has the ten and west the queen, I can lead the jack from hand, forcing west to cover, then finesse east's ten.)
Another example:
K9x
AT8x
I can always make 3 tricks in this suit, double-dummy, by first leading low to the nine, then, if that loses, guess where the second honor is, double-dummy.
There are cases where other holdings are over-valued:
Kxx
QJxx
We get three tricks, doubly-dummy, if the suit splits 3-3, or if the ace is in the hand with the shorter suit. But any single-dummy play must choose to play east for the Ax holding if he wants to keep the chance for 3-3 split.
Other problems are just that certain types of questions aren't really suitable for double dummy analysis. You might ask, for example,"If partner opens 1NT, when should I inivite with a natural 2NT response?" But the answer to that question is made trickier by the fact that the 1NT-2NT-3NT auction tells more to the defense about your partner's hand, and the defense knows on the opening lead that you are likely to have a borderline game. You've basically told the defense that you have about 24-25 points between your two hands.
The possibility of using single dummy data is tantalizing, but it provides its own set of statistical problems.