Frost giants important? Sorry, but I haven't used a frost giant in 3.X at all. They're... boring for me. And that's the point, some love the frost giant, some hate it, some ignore it.
But I hardly think that a frost giant is very iconic, compared to things like mind flayers, modrons, or drow.
In the end, the most important thing is: Good, usable, sensible monsters. If the MMI can deliver that, then I'll buy it. If not... well, then 4E probably won't deliver at all.
Look at it from another point of view: Will you need frost giants in your very first 4E campaign as a totally crucial, irreplaceable element? I dare to say no.
Then... when will you reach an appropriate level for frost giants? Let's put them on 9th level (power-wise), then it will (at a 1 level per month rate, that many people see as far to fast) take about 9 month until you actually need frost giants. Until this point, it's probable that you get your hands on some frost giants. On ENWorld or another home brew forum.
And as a "bait" to buy new monster manuals? Well, you don't have to buy'em, yet the MMs and different creature collections, Tome of Horrors, Monsternomicon, Denizens of Avadnu are all bestsellers. Or use the plentiful free stuff all over the web. Or just switch fire to cold for the frost giant.
Cheers, LT.