Because we have some form of cover for Torp in the form of Eccles and, to an extent, Allen.
We have no-one that plays the same way as Grimes. And it's all very well saying 'he never gets injured'. Same argument was used for O'Hare. Until he did get injured. To not have someone there to fill that role is just piss poor planning. We've also noticed that Grimes hasn't been quite up to his usual high standards last few matches because he's played so many minutes fatigue is showing. And fatigue can be a precursor to injury.
And frankly if we're looking at being PL next season, Grimes is only going to be good enough as a rotation option. Plus his age also means we need to look towards the future. Not saying we can find or afford that upgrade in this window but if the option is there we need to seriously look at it.