Mostly it is only when you get hit by single-target abilities. It is a very nice mana gain on Yor'sahj and Spine due to all the adds, and also a decent sized mana gain on Morchok and mana neutral on Warmaster. Ultraxion, Hagara, Zon'ozz, and Madness you would be better off not glyphing it.
Glyph of Riptide is most useful in spread fights in 10 mans, where HR will not do much and it will be hard to get full use of CH without the glyph. If you use RT as a triage HoT instead of spamming it on everyone when no damage is going out, it will not OOM you any faster than spamming GHW on people who take damage.
In a stacked fight, you are right by saying stick to HR. Glyph of Riptide is an optional way to change your healing style, but if you are looking to push progression or top the healing charts, it will be necessary on certain fights. This is from a 10 man perspective, on 25 man it is doubtful there will be any fight where Glyph of Riptide is necessary to be competitive.