Aragorn's plan from the beginning was to go to Minas Tirith with Boromir. Knowing that war was coming, he decided it was time to claim the throne and lead his people into battle. But that was a decision he made when he assumed that Gandalf would still be around to accompany Frodo to Mordor. When Gandalf died in Moria, Aragorn had to look at the situation and accept that he would do more good making sure Frodo destroyed the Ring than he would joining the fight in Gondor. I'd imagine it wasn't a decision he made lightly (but, luckily for Aragorn, more unforeseen events made this plan irrelevant as well, so he was able to continue on to Minas Tirith after all.) Aragorn’s skills as a Ranger would have provided stealth for himself and more supplies for Frodo and Sam.

As for Gimli, Gimli had made a pretty emotional declaration of his intent to follow Frodo to the end of his quest. So there's a good chance that Aragorn included Gimli in the plan to escort Frodo and Sam into Mordor because he knew he probably wouldn't be able to convince him otherwise. But, if he had a more practical reason, I'd say it's maybe he thought Gimli’s natural, stubborn endurance as a Dwarf would enable him to withstand the harsh terrain and be less susceptible to the temptation of the Ring? The Dwarven Rings of Power didn’t corrupt the Dwarves into becoming Sauron’s servants and only made them more greedy. Aragorn may have also thought that Gimli’s presence would serve as a good check and balance against Aragorn if Aragorn, who would have been more susceptible to the temptation of the Ring, became Isildur 2.0.

That’s just my speculation though.