Even when you hire freshers, we should not ask the candidates to sign without the date of joining mentioned in it. If you are asked to sign such a document, I would say that the company is not a good organization to work in. It is true that freshers, who do not have any previous knowledge about the system in a corporate setting, will accept the offer. Later, they will understand that some date later than the actual date of joining is put in the service records, PF, etc.
Again, employees who do not find other options may also join such companies. This will lead to a lot of disputes in the future. Why should we get into these when we have other opportunities?