1. foreign wordstock (which is just the same if you learn Spanish or French or modern Greek)
The latin and greek wordstock in modern languages come from ancient latin and greek, not from their modern counterparts. It's like if you were trying to understand russian wordstock by learning polish (or even silesian). Especially French is a bad example (it;s perfect for understanding the french wordstock, however!).
2. reading some books in their root tongues (ok maybe this part they win but not by much)
That's definitely a plus!
So you gave reasons to learn ancient greek and latin... which is two more than for learning esperanto. :P
Some do speak Esperanto natively btw which is more than can be said for "Latin" or "Dead Greek".
hmm I stand corrected but I wouldn't be surprised if Star Trek fanatic couples were learning their kids klingon (making them native speakers) either.
Yeah, esperanto was made to combine all the language into one.
So it is like learning 1% of every language.
Learning "1% of a language" is like learning none. Nor "combining all" makes it comprehensible to anyone... especially if completely different words in different languages can sound the same. Unless you are basing off already similar languages, like interlingua for romance, interslavic for slavic.. or standard german for modern Germany's dialects and languages. Otherwise it's a big mess.