The idea is that icy veins outside of bloodlust/heroism is a dps gain over using it during bloodlust/heroism. Using the glyph allows you to use it during heroism/bloodlust and get a dps gain. So if you wanted to maximize your burst during bloodlust/heroism and not care about it the rest of the fight, the glyph is useful.
First, go read Kuni's guide (it's a sticky).
Second, the problem isn't with Frostbolt going under 1s during Blood. It's just that without the Glyph, all instant spell will be GCD capped every IV.
This is why the IV cap is around 4k and not 6k haste.
just t not, that in T15, we will be GCD cap for instant very haste proc and passively in T15H.
Read Kuni's explanation in his guide. Also, be sure to understand that is assuming all haste buffs in a raid, therefore, if you do not have the 5% haste buff, you will need to adjust accordingly.