The problem is that peoples incorrectly consider something as an exploit, while technically it is not. I dont know what is a true definition, to me exploit is something thats not possible normally and player must put some effort or knowledge to do this. Like casting epic mage armor with only 5 wizard level, taking bane of enemies without 21 ranger levels, casting a normal spell while polymorphed, uploading a selfmade character on server, passing through door and such.
Using a single monk level, single shadow dancer level or casting metamagically enhanced ice storm is not an exploit. Neither (ab)using the caster level calculation for spells cast in polymorph is. Game normally allows this and player doesnt even have to be aware something is wrong. Why would be? Its either bug or an intented design. The game is based on DnD rules, so not everything that doesnt matches with the rules is wrong.
Whether is it bug or intent is different discussion, I could find it out in hardcode if I would want but its waste of time to me - I dont think that casting metamagically enhanced icestorm is a problem. It might as well be intented, because each quickslot has a class position assigned to it, so they either overlooked that class have to be compared or intentionally ignored it. Another reason this being intent is that the spell cast from quickslot like this uses a caster level of the original class no matter of the position, thus skipping the caster level bug. So if you make a 21monk 9druid 10shifter, you will be able to cast icestorm with either 21level or 9level. So you can normally make a 28 10 2monk build and start with monk because the caster level bug happens only when casting the icestorm quickslotted from the polymorph.
The caster level issue is definitely a bug. Normally each spell has a class it comes from. However polymorph attained spells doesn't because the shifter class is not a source of the spell, the polymorph is, thus no class is found and in such case it retrieves a value of first class. This means that very possible intent was to return shifter class level as caster level.
Truth to be told, there are few exploits with this that I fixed in my community patch. For example, it is possible to cast maximized ice storm without having the actual maximize spell feat and more...