I'm split on this. Lots of reasons I'm not that keen:
He's not worth anything like that sort of wage - a free transfer negates some of the financial impact, but not the knock-on effect on others' wages or the media pressure.
He's a bit of a prima-donna and doesn't necessarily justify it yet, and we already have a star midfielder who does merit the status.
He can't or won't do the one thing we really need at the moment in our midfield - replace Matic as a positionally disciplined, out-and-out DM.
Looks to me like someone who benefits from playing 90% of his football against grossly inferior opposition, keeping him fresh and rested for those occasional big games. Is he going to struggle with the grinding intensity of the PL?
But he does have one big upside, which is that he's someone who could come in and get plenty of football next to Pogba - in Herrera's position or Matic's - but could also cover Pogba in a way that no-one we currently have can, providing that forward momentum, quality, physical presence, flair etc. Anyone else we bought to cover Pogba would fall into that star-player-backup catch 22, that no-one with the quality to do the job would be happy to sit on the bench most of the time. Rabiot's relative positional flexibility avoids that problem.
EDIT: And I guess you also have to say his style of all-over-the-pitch defending would suit Solskjaer's apparent preferred level of press (that is, fairly high a lot of the time but flexible and not gung-ho like 2011 Barca or Klopp's Dortmund.)