I don't think it's just to make ELF happy, it's a smart business move. Why let half the guys in SJ sit and do nothing, they're missing out on money making oppurtunities. I know they were planning to bring back SuJu-T, but I think because Heechul is looking to do another drama, they replaced him with Yesung. Which actually works out really well because then they all have something to do~ Hankyung, Siwon, Donghae, Ryeowook, and Kyuhyun have SJM, Kibum and Heechul have acting, and then Leeteuk, Yesung, Sungmin, Eunhyuk, Shindong, and Kangin have SJH~! So they're all busy with stuff to do
Which really makes me happy, because I was feeling bad for Yesung since he wasn't in an active sub-group and the others that weren't in SJM were either acting, or were coming back with a 2nd SJT single. But this way they all get love~!!!
And this way they're all, even though they're kinda doing individual activities, are still promoting SuJu as a whole ^^
Anyway, Yesung fit in when the 7 of them did Haengbok at HB~! So I'm really looking forward to their music~!!! But I'm gonna miss Heechul's antics on stage ;.; But more drama's are always