I wouldn't be worried if I were you. Having your mirror images put a debuff on the target is a lot different from you yourself having to spend talent points on something and make sure your rotation keeps it up there.

Since MI is available to all mages, I think this is just to make sure that the actual talented versions of the debuff remain better than what can be acquired by any mage for the cost of a glyph.

Yes, but the effect they're giving to the images has the exact same name as the frost talent for the same purpose. Also, you'll note that Warlocks are getting a change that will make shadow bolt offer a 5% crit increase to the raid.

Add in the fact that they wanted to make this debuff "less of a unique snowflake", if I got the quote right, and I think it's a very good guess that the overall effectiveness of this debuff is being reduced and spread around.

Arg, and I meant to add: fire mages have an improved scorch glyph to get their debuff to a full stack in 2 casts. This seems a lot like it enables frost mages to pop mirror image and have a full stack in about the same total time spent, so that also evens it out a bit.

I don't see where an arcane mage would choose this over several other glyphs, especially if there are other mages or warlocks in the raid.

Now that Frost is supposed to become a more viable raiding spec I think that's totally in line with their current design direction. I guess time will tell!