Those games coming to PlayStation would generate more sales for Microsoft. We already know Xbox games are coming to PlayStation so its really inevitable at this point, its just matter of when will the other games be ported. Xbox is struggling and there's simply no coming back from that.
By announcing Gears of War coming to PlayStation this year, they could lessen the negative press that Xbox is collapsing/closing companies/firing people. The attention would be directed to more Xbox games coming to Playstation, which will generate money anyway.