I think there should be a popup window that lets you know the cap hits your team will take if you make a trade. So once you have a trade in place you hit offer, before it actually sends the offer you can see the cap hit (dead cap) and an overview of the trade before it's sent. I think it would help people look at the entire picture of a trade before they pull the trigger. :)
It matters when you take over an AI team that has resigned all the players to terrible deals. I usually only have teams i've built from the ground up so my cap situation is solid but i recently took over a reclamation project that not only has a horrible roster but it's filled with horrible contracts. So i'm making lots of moves and the dead cap is very important.
You guys are more experienced than me, but if you want to know the trade ramifications just go to cut the player, it will show current and next yr cap hits. Just cancel the cut and continue with the trade.
i know that i can figure it out manually that way but for the average player they just offer trades or accept without all that work and then deal with the results after. Just seems to be an easy fix so you can see how the trade will effect your cap. It's there for a cut why not have it similarly for a trade?