Do you want quality sales people? Sales people that will devote themselves to all aspects of the sales and account management process? Even doing those parts of the job that may not directly pay them any commission? If the answer is yes, and it should be, then there has to be a reward for such. Otherwise elements of the whole sales process will be at best done in a shallow way, or at worst, not done at all.
I understand that there are some Companys that work on a commision basis only for their sales people, but, in my eyes, such sales positions are very much at the lower end of the profession. Often people take such jobs as fill ins with no real intention of furthering such a position as a career growth situation. This means that not only are functions dealt with at the shallowest level possible, but, the sales are probably not of the desired quality either. What is the fail rate of such sales, a little further down the line after the close? How many of such sales have a high problem factor after the sale is completed, simply because some sort of hard sell technique has been used, because the salesperson can only see the commission reward on the close? I put forward that such sales have no long term value and are not much use unless you sell double glazing or vacuum cleaners.
Certainly, if you want your salesperson to devote themselves to the long term close, within say consultative selling, where partnerships are made for the long term revenue return; there is no place for commission only sales positions. The salary is there to bolster living expenses whilst the salesperson populates and farms his sales pipeline.
In addition, a good Sales Manager wants to build a team where the team members see the department and the Company that they are working for as a career. This again will assist with diligence within the process. For this to be a reality, where people are grown to become team leaders within the department, time has to be taken for training. This time should be covered by salary, as otherwise the individual taking the training will not focus upon it properly as they will be thinking of the time lost trying to earn some money.
Lastly, most sales positions have a degree of account management attached to them. I would want my sales people giving all the time necessary to my Customers if it keeps the accounts live and happy. Again though, some of this activity does not produce actual revenue, do you think a commission only salesperson will want to devote time to this? I think not.