It takes a bit of time to wash out the spent grains. It can be used for that purpose too, but you also generally have 2 or 3, if not more, hop additions. It'll take some time to chase down the bag, open it, and drop these pellets in. It's far more easy to have separate bags.
I had reusable hop bags, but one tore within several months. But, with all of the other things to clean, I gave up on hop bags, and just use muslin bags.
And what I do with them prior to dropping hop pellets in is to stretch it out a bit and shake it over the trash can, which will get rid of the few strings/lint.
I have one reusable hop bag now which I use for dry hop additions only.