The most prominent complaint that lead to this was people spiking inactive accounts (i.e. defending the raids sent on other unaffiliated inactives). Another concern being that it was easy to have private farms/multies to raid (without alliance pushing restrictions), and then reinforce them from the alliance when they were attacked/raided by other players.
For supporting players in other teams with defense as a spy... You wouldn't be able to do that anymore anyway, since reinforcing troops are rewarded with the defense points now - so would give you away if your points go up (unless you're sure you'd stay out of top 10). Besides, the biggest impact a spy can have is with information, not sending his defense to the alliance he actually plays for, so I don't see how it impedes spying too much, like you imply that it does.