Blockchain transactions will prove nothing, because there is no one from Bitcoinica to prove that the addresses belong to them. Soon their liquidators will post reports regarding all claims, which hopefully will be an enough proof that the funds were in fact there.

The rationale for investing is that bigger services either won't sell to anybody, or the price is impossible for investors to pay. In case of Bitmarket for relatively small price the potential is quite big.
Please keep in mind that MtGox paid ~17000 BTC for bailing out bitomat.pl's users, which was MUCH smaller (only traders from Poland), so here it's not unreasonable. Lastly, I'm offering my technical support at every stage of investement, even months after that, until all is repaid.

Update: I've implemented deducting the fee from "on hold" balance if a user has one, as suggested above.