I'm getting most of my transactions "malleated" right now, so what? The original transactions never confirm, but the "malleated" ones do and they are registered correctly by Bitcoin-QT. You just see all your transactions twice, one is confirmed and the other will never be, so what? Balance is displayed correctly by QT because the standard implementation handles correctly malleated transactions.
What could be the motivations of whoever is doing this?
To exploit an MtGOX-like bug on any exchange or system that may happen to have such a bug?
There's no "exploit" unless you use a custom software that uses "transaction ID's" as the only element to check transaction, and then you let someone to "social engineer" you by making you believe they never received the money you sent them.
If you use the reference implementation (bitcoind/Bitcoin-QT) your wallet will show BOTH transactions (the "malleated" and the original one) and will display the CORRECT balance, so there is really no way for you to be social engineered. If you use a custom implementation that only cares about txid, then probably the "malleated" transaction won't show up in your client/accounting system, leaving the door open to a social engineering attack.
TL;DR: whoever is trying to "malleate" as many transactions as possible is doing it either for shit and giggles, either for trying to spread some panic among novices and thus induce panic selling.