Elsom is a mixed bag in terms of value. Yes, he is a one of the best back-row forwards in the world, a proven, consistent performer when fit. Brian O'Driscoll said without equivocation that Elsom was the best player he had ever played with. However, "when fit" is the operative term here.
Elsom needs special "handling" and there must be meticulous oversight of rest vs training vs game-time for him to avoid stress injuries to his leg muscles in particular. I'm not sure where a S15 teams gets value here. A team like Leinster, whose season is broken up by the 6N's and where the less competitive Magners affords more opportunity for rest for the more intense HEC comp, was ideal. But the big money is in France, and Elsom has stated he has little interest in returning to Dublin. He has been there and done that.
If Stade Francais' financial problems are behind them, I can see Michael Cheika trying to lure him back up north; he knows how to get the most out of Elsom and by all accounts they have a very good relationship. But SF would probably be the only team in France where he would get such sympathetic handling.