I was reading about this topic yesterday about the positives and the negatives of having Steem on Coinbase exchange. I've come to my conclusion of what I think is best for both Coinbase as well as Steem. 

There are good things that could come of coinbase adding steem to it's exchange. So much more publicity and users will make their way to steem. The value of steem would skyrocket within time because it'll be much easier for people to acquire steem. With the upcoming SMT's coming, that will change the price even more going up (hopefully).

However on the other hand, having steem on coinbase could cause some problems. We all know that Steem is STILL in beta. Steem, ever since it was created has been in beta stage. That means it's not a finished product yet. Having a massive flood of users joining Steem expecting it work great may disappoint some. There's problems that will occur from time to time in the development of steem. 

That's just my two words of my opinion. Honestly, I do not want Coinbase to add Steem. Not just yet, I think it's too early.