You are so lucky being close to all those tournaments!!!
I am not a big fan of grass either, but every player who has aspirations to reach the top should play all the big tournaments.
I always preferred clay, but I never completely understood the players who skip the grass season... and I'm pretty sure that with some time Del Potro could become a good grass court player.
Yes, he is tall with a big serve. and he moves well. I think you could be right. Lots of the Argie players seem to have a mental block against it, but JM has already said he prefers harder, faster surfaces, so with some work and practice, he could do well