There is no harm in doing so after the election. By then, it is too late for the Coakley campaign to launch the inevitable (Scott Brown: REPUBLICAN) attack ad.
After the election, you can't hide the money, anyway. So you might as well acknowledge it. You just don't do like the morons over at the DSCC, and issue a press release along with the money, because that puts the national party in the middle of a local race. The only reason to do it is because the Democrats are too insecure and self-promoting to let an apple-polishing opportunity go by. It is good to see that Steele has more sense than that.
Steele mentioned RNC participation before the election was held, although it was probably too late (as you note) for the DEMs to produce and distribute attack ads based on Steele's statement.
But the general principle of "discrete giving" is no more. Agreed that the transaction can't be hidden, on account of reporting requirements, but it need not be touted at any point.
My sense is that Steele touted the giving in order to gain respect (and potentially money) from conservatives who have little use for the national party apparatus.