I don't mind former players coming in as assistants. I would, however, be against appointing one of them to be groomed as Arthur's longer-term replacement as first team coach.
This is because coaches inevitably have limited shelf-lives at a given club (Bellamy aside). At Leeds right now the knives will be out among certain sections of our support within 10 games if we are not winning on a weekly basis. We don't want to see this happen to players that were once favourites, all the more so if it is their first gig as first team coach.
Sinfield did the right thing in quickly moving upstairs and exiting the club after Agar had steadied the ship. Contrast Cunningham and Wellens, two giants of recent history at Saints who look set to be remembered as coaches who just weren't up to it.
Others have been more lucky - for example, McNamara was heading the same way at the Bulls but he managed to resurrect his career by hanging around the NRL for a few years as assistant and doing the national coaching job when we had a handful of decent players who could hold their own on the international stage (e.g. Burgess, Tomkins, Hall etc). Now even he is surely coming to the end of his tether in Perpignan.
A sensible former player stays on as assistant permanently (O'Loughlin?), or moves on to another club if they want to be a head coach (IMO).