Summoner can sometimes take place of whm in experiencing parties. But in very dangerous areas of lvling they can only be efficient back up healers especially low lvls. Because they dont have raise before lvl 50 with whm subbed. And they dont have the big cures spells or benediction.
Summoners in NM, HNM or any big lvl battles will never work as main healer only buffer. Other than that they can be decent healers at times.