It depends on what a beginner want to be, I guess. I mean: if one loves speed chess, I think he can improve as a blitz player by playing blitz. If you love slow chess, maybe playing blitz won't do you any good. I say "maybe" because my judgment is based only on my personal experience, maybe for another it could be different.
To answer the initial question: Personally, I’m not interested in blitz mode. Also for me chess means deep thinking, position's analysis, long calculations ( although, as a beginner, I am still lacking in all three of these characteristics...). So, I’m interested in getting better at rapid and classical modes; and playing blitz does not bring me any improvement.
But for a reason I find it useful to play blitz games: it's a quick way to get and accumulate information about the openings I use, trying new lines, seeing which ones work and which ones don't. This is essentially why I play blitz, even if I don't enjoy it.
"For improvement, Rapid (15 | 10 or longer). Classical (for amateurs), albeit useful, is not as efficient relative to what you would learn in a longer time-control Rapid. Therefore, amateur players should find a longer Rapid sufficient, as they will not comprehend the nuances each move carries in Classical."
What do you mean by Classical?
There is no such category on this site. Just a huge rapid pool of over 15 and a half million players encompassing everything from 10/0 to 60/0 which is therefore totally meaningless.