It all depends on what the incentives reward. If you merge the two, and maintain higher rewards for sales, then you’ll get lots of new customers, and they’ll be out the door just as fast, when they realize they aren’t gettng good service.
If you incentivize good service, and skimp on sales, it will take longer, but eventually customers will find you (after getting tired of all those other companies that didn’t care about service).
It is practically impossible to design an incentive system that rewards desired behaviors appropriately, if they have to be the same person. However, there is some value in having salespeople do service, so they can understand what their customers really want.
Actually, I think it can be good to merge both teams, even if the jobs are separate. Communication between service and sales is so important. That could facilitate the comunication.
What if the two functions hate each other? Service is complaining that sales over-promises. Sales thinks that service is a bunch of whiners. Would it be better for them to have to spend time together, and hear about each other’s problems, or would that create too much acrimony, and so you should keep them very separate?
One argument in favor of separation is that the functions are quite different, and it would be counterproductive to have folks spending time in meetings that had nothing to do with them.
Overall, however, I think it’s better to combine them. Sorry.