Flip-side of DCA

I'm a novice for investment fund, so what I got at from my limited experience of investment could well be inaccurate. I'm still anxious to share my opinions notwithstanding running the risk of being so wrong that I could become a laughingstock.

Financial planners are always coaxing people into investing in mutual funds through dollar-cost averaging (DCA). The reason they hold seems quite strong and persuasive--that can help you avoid buying too much when the market(NAV) is high and too little when the market(NAV) is low. As a matter of fact, the reason is one hundred percent accurate; it's self-explanatory if you have basic mathematical savvy. But what is the flip side that few financial planners would mention to their clients? From my monthly account statement, I sadly found that not an installment has a lower net asset value than the first installment. Yes, it's "averaging" my investment, as DCA denotes. But in other words, it's more like "diluting" the returns I should have obtained if I had adopted lump-sum investing (LSI) strategy instead. In only one situation, I guess, can DCA beat LSI,i.e. the market has been plunging for a long period so that you cumulate lots of your funds at quite low NAV each fund unit.