SIP maths vs psychology

There seems to be a persistent and loudly expressed belief among some people that investing through an SIP is useless. The most frequently expressed opinion is that when you have been investing for a long period, then the volatility of your holdings is practically equal whether you have invested through an SIP or in a lump sum. There seem to be a number of blogs, Facebook posts and opinions floating around on social media expressing this. There are a number of mutual fund distributors too, who try to convince their clients of this, for reasons that are not hard to guess. The conclusion that is generally drawn is that SIPs are just a sales trick and that there is nothing useful to be derived from investing through one. This accusation, if I can call it that