The reason dispel is considered the best glyph, is because dispel, as a priest is your most used spell in general.
Lets say I cast renew, without the glyph, it ticks for 100. With it, it ticks for 125. Awesome and what not.
Over a course of a renew, you're still doing the same amount of healing, albeit it in larger margins. The idea is that, the renew costs 114 mana, for only pure healing.
While glyph of dispel will remove whatever dumbass Hunter's Mark, Fear, Psychic Scream, Immolate, Corruption, Web, Freezing Trap... Etc etc etc. AS WELL as give your target a heal on top of that.
Our FC in premades sits at 2.8k(Or some shit), While I'm being useful by removing shit, and healing him with dispel for 84 a pop, I can ALSO have renew ticking. Which still does the same amount of healing Unglyphed, just over another 3 seconds.
Hurka durr, renew if you're a bad priest who doesnt make use of dispel. Or Dispel, if you use dispel.