IMHO I2a2 (old I2b), or at least some of its subclades, matches Beaker Culture a bit better than I2a1a. The STR dating and distribution of I2a1a suggest that it spread earlier and more southwestern than Beaker Culture, especially if we assume that Beaker Culture formed with influence from Corded Ware. The point of contact between Beaker Culture and Corded Ware is very close to the center of diversity of I2a2, and the age of a lot of its subclades, especially its Western ones, are only a bit older than Beaker Culture, suggesting that Beaker Culture helped them spread. However, Beaker Culture appears to extend beyond I2a2's reach, suggesting that the Beaker admixture contained something else, maybe some I2a1a and G2a that got picked up, and others (R1b being a big wildcard). Ancient DNA will hopefully resolve these ambiguities... either way, I don't think that predicting Beaker Culture's Y-DNA will help anybody's argument at the moment.