That is not necessarily the case.
Sometimes you have teams that are all just heavily piled on top of windmill, and your team isn't get to get them off of it without getting completely wiped.

At times like these, you can instead go bottom, and just 4-5 man their bot point.
This will force the top defenders to move their complacent asses off the top point and cause a bit of chaos or confusion, and you can use that opportunity to head back up to top while they are trying to hold bot and get it that way.

OR, if the defenders stay top, you get their bot point, and now *your* team is the one with a 3 cap, and the enemy team has to be proactive instead of lazy.

There're simply situations where you have to make the enemy team who is turtling at the windmill react to what you're doing without their tower advantage. You can't just go walk into a 4v4 while they have the tower advantage - that's flatout suicide. The smart thing to do is to make them react to you and hopefully their coordination is such that an opportunity presents itself to cap a tower.

For example. Often times at the end of the game you're winning 20:300 but they have 3 towers and you have two. Do you waste time trying feebly to take back windmill that they're just camping? Or do you send 3 extra players to the bottom covertly and cap it really quickly and end the game right there? I often call for 5 man all-ins in that scenario and it's won many games.

Like Ra's al Ghul said "Theatricality and deception are powerful agents."

Sometimes what you want to do is confuse the enemy. You cap their bottom point, suddenly you get one or two guys recalling to stop you. Now, the battle up top is a 3v2. Even just neutralizing their bot point can cause enough confusion. Plus, I seen games where someone does this so well that everyone starts to get frustrated and lose all sense of coordination. Suddenly, you win the game you thought you were going to lose.

The guys above me covered the reasons you're incorrect pretty well. I would like to add that 2 people on their bot require usually 4 or more people to take it back because they have to fight you on point. It's really not that simple to get that point back for that reason. Meanwhile you have 3 teammates who can roam and take other points.

Everyone above pretty much described why you're not always correct. Upvotes all around.

... And its time for our next segment called...
Did you know?

Did you know no matter which points you hold, the game will tick down the opponents nexus health if you own more than them?

Everything said above here is true. Dominion was not designed with 5 points so that only 1 was meant to be fought over. It is a game of timing and resource management. Suiciding a turtled Windmill is a surefire way to lose many a game.

Funny thing is, I was just raging about my team doing just what the OP suggests last night in dominatedominion chat. Enemy is camping Windmill with Malphite and Alistar... we have a fraction of their hard CC... yet my team is convinced that taking bottom is a losing strategy.

USUALLY what happens... is we have three no problem... so everyone abandons top to jungle or fight over bot, leaving me up top alone to guard WM against 2-4 people. That's about 30% of my games. Now that... THAT makes me want to punch kittens.

Of course there are times where going bot is a good idea... even a GREAT idea... but those are the times people refuse to do it, lol.