Formative Assessment and Summative Assessment are the two types of assessment modes in any educational system.The former denotes periodical tests while in the latter each student is assessed at the end of a year or standard.
Another way to look at it is that there is no distinction betweeen formative and summative assessment. They are just the one thing - assessment. It helps if you think of assessment not as 'how well they can peroform what they have learned' but 'what do they know now that they didn't know before' In the latter, every piece of assessment simply helps us maker a judgement of what a student has learned. If a judgement of relative worth is required, then the one who has learned the most at the most complex level is the better/best. Of course that also means that you need a set of descriptors of those things that are valued in the form of a contiuum.
However to answer your question more directly: If you don't distinguish the 2 forms of assessment then you can do without it. If you do distinguish them, then doing without it will resulkt in a poorer form of teaching.
Thank you, for your well described answer Mark. Yes, I do agree that any form of assessment has the outcome in terms of what a student has learned from a predefined curriculum.