I don't want to abuse anybody. Here are just my thoughts about it.

1. Trade system leads to cheating
Really? If you give your sailors for no credits just for a good sake, and believe that you will receive credits later, doesn't this mean thta problem is in you, not in trade system?

2. Bad influence on NF store, medal store and olive store

Many people say that trade system dosn't give motivation for people to buy olives, items, etc.
Okay, lets look at situation of mine, for example. I am a student and don't have enough money to buy olives. However I can buy boosts or vets in game for credits (I think it's not a secret that many players did it).
But look: if I buy vets\boosts for credits, the person who sells them to me buys them for olives. In fact, this person spends money and invest them in the game anyway. So it looks like a profit,doesn't it?

3. Trade system = no desire to play and grow up new crews

Okay, let's look at the example:
Imagine I have MN bombers and want play in mn fps set. I have no opportunity to buy elite fighters as well as vets. So what is the reason for me to grow 12 stats fighters with 100 vets max, that will die against elite or super elite fps? With trade, however, I can exchange my bombers for quite good elite fps.

This may be applied to bb crew as well.
So, trade makes it fun to play, because you can change sets without pain.

4. Newcomers complain that they have not enough credits
I remember old days when you could ask mates to help with credits, so you could buy your first bb 6 or cv 6. Now newcomers can't afford buying even bb 1( Isn't that bad?)

It's all just my opinion. What is yours?