Having a hard/standard cap and limiting or eliminating the ability to pay on contracts will over time increase the value of cap space and potentially lower the annual salaries since GMs will know they are on the hook for the salary they hand out in FA unless they can find a trade partner to take on the salary or offset it with contracts in return.
This only works if re-sign values are fair, give the state of the salary cap and amount of total players per roster.
If re-sign values are just thrown out there, or if they are in line with the real life league, then it doesn't make sense that we can't pay money on contracts.
Might make it harder on mod's on the outset to set the fair market value for re-sign values, but overall in the long run might cause less spreadsheet headaches.
That said, it has worked well where when say you pay $10m on Player-X to make a deal work, that $10m always is paid on him from the original team. So if he has a $20m contract and $10m is paid for when he gets traded - that $10m paid goes with him. It's not like you can trade for a player and get money paid on him, and then turn around and flip him and not pay money on him but have that extra $10m in salary cap because someone else paid you in the original deal. Makes it easier for spreadsheets that way.
But overall there are some big big salary contracts currently that are pretty well un-tradeable unless you're sending another big money contract he other way. Sometimes the only way to move those enormous contracts (or bad contracts) is to help out with salary.
(Case in point, Brian - you in BoD having to pay money when trading a guy, to get rid of some of your overpriced players. You still get value for them, but there's not way you're trading a player without paying money one him to get a deal completed).
