I don't really see that it makes much difference. You have two options:
#1 - drop a player before the trade processes, so that you don't have an illegal roster when the trade deadline hits. This gives you a few extra days to think about who you would drop. If you know right away, then drop the guy when you accept the trade. If you want to wait in case the trade gets voted down, then just be sure to do it the morning the trade is supposed to process.
#2 - if you want to avoid all that, just make sure you are trading an equal number of players. If you're going to drop a guy, counter offer the trade to give away your originally offered player and include the player you are dropping. This is essentially the same as dropping a player for you and it doesn't hurt the person receiving your dropped player, as they can either hold that guy or drop him for someone else.
The disadvantage here is that you have to remember to drop a guy or that you may drop a guy early and then the trade doesn't go through. There are easy ways around that though using either of the methods above. On the flip side, the current setup allows you to think about who to drop for a few extra days. Also, if you force a guy to include his dropped player in the trade up front, then that team does not have the option to drop their dump player while the trade is under review. What if the trade doesn't go through and now you've missed two days of potentially picking up a replacement for the guy you were going to drop?
Here's an example: You have J. Charles and he goes down. Someone offers you two RBs for one of your solid ones. You agree and intend to just drop Charles as he no longer has any value. With the old method, Charles has to be named as the player to be dropped and is locked up for two days. If the trade doesn't go through, you still have Charles two days later. With the current system, maybe you could drop charles and get McCluster to replace him early in the week. If the trade looks like it will go through, you can then just drop McCluster shortly before the trade review deadline. If the trade doesn't go through, then you didn't waste two days and risk someone else potentially picking up McCluster.
I actually like the current system a little better as it gives you more options, but I really don't think it's a big deal either way.