[the phantom]

He joined for the SOLE purpose that the Fellowship's road, and his road to Minas Tirith followed the same path for a long ways. I see that as a wrong reason of joining, only joining to head back home

What's wrong with that? You realize he coud have left for home before the Fellowship set out. Elrond could've found a horse for him and Boromir could've ridden home the same way he came.

But he didn't. He travelled with the Fellowship and made himself useful. The Fellowship probably would not have survived Caradhras had Boromir not been there.

Elrond specifically said-

The others go with him as free companions, to help him on his way. You may tarry, or come back, or turn aside into other paths, as chance allows. The further you go, the less easy will it be to withdraw; yet no oath or bond is laid on you to go further than you will.

Boromir didn't have to do anything, yet he travelled with the Fellowship and helped, and in the end died trying to protect two of his fellow members.

Boromir deserves honor for his service.

and we see Aragorn willing to follow Frodo to the end

But it was not in his plans until Gandalf died, and Gandalf's burden was laid upon him (but not laid upon Boromir).

And SPMan said this-

I do wonder whether Boromir would really have left Frodo in the event that the Company had been faced at this point with the decision of going east or west.

I also wonder this.

Not to mention, how do we know that part of Aragorn's willingness to follow Frodo and the Ring (despite the summons to return to his kingdom) wasn't due to the lure of the Ring?