Technically, yes it is pointless but, if you're amping them instead of using the headunit amp, it is worth having a crossover in there. In my opinion anyway. The tweeter is where crossover is effectively, as if you take the tweeter out there are a second set of wires that are for the woofer, so where the sound comes from, it travels to the tweeter, then down to the woofer.
Now what I'm saying is, by putting in a new set of components, you'll going to have to cut the block wires that are plugged into the tweeter and woofer in the door card. Reason being the majority of aftermarket speakers have speaker terminals instead.
Now in doing this, how does the sound get to the woofer? Yes you'll have a speaker wire connected to the tweeter but what about that loose woofer connection? That's why a crossover is essential. As you ignore the cable that originally ran to the tweeter, if you're amping them as you'll now use the speaker cable coming from the amp, take this into the input of the crossover, then there are two outputs, 1 for the tweeter and 1 for the woofer. The new tweeter will come with a cable attached to it most likely, then it's just a case of wiring it in, same for the new woofer.
Hope this makes sense?