To be fair, it didn't seem unreasonable to send only one cube. The ship wasn't even surviving the attack from "Q Who?". If Q hadn't flung them back to and away from the Borg, they have lost. The Borg were obviously technologically superior. To back that up, we have the battle of Wolf 359, where one cube took out a small fleet of starships and suffered to apparent damange when the Enterprise cauhgt up to it. Plus, if we use passed Trek as any indication, there's like, never a friggin' starship around when you need one at Earth or even the immediate solar system, unless you're trying to steal the Enterprise.
It's only later shit Trek ideas of the Borg that made them so weak, including the First Contact film where apparently all you need to do to defeat one cube, is fire on one certain spot of it, even though previously established dialogue in TNG indicates otherwise. Or be the U.S.S. Voyager.
If anything, why did the Borg even want to assimilate the human race? What possible technological advantage did humans have that the Borg had never encountered before in all the other races they assimilated? Oh no, they can modulate their sheilds real quickly -- I guess processing speed is like an old 1998 desktop computer.
Also, why even make the out of the way trip to do it? Why can't Earth just be assimilated when the Borg get there naturally? Surely by them the Borg would have evolved so much so technologically that the Defiant would have been utterly useless. Was the Borg Queen just a petty vengeful bitch who wanted revenge no matter what sense it even makes? Assuming we are buying the pure Grade A Balonium First Contact gave us about the Queen being there all the time for "The Best of Both Worlds".
Not to forget the inane shit about stopping the human race from even achiving warp by destroying the Pheonix and assimilating Earth then. What the in possible fucking goat Hell would that do for the Borg? The Federation would never have developed the technology the Borg wanted to assimilated. The battle that instigated the travel back in time would never even have occured, wiping out the events even happening; and don't hand me an alternate timeline argument; if the Borg wanted to assimilate alternate timelines, they could do that all day long.