I must admit I am a bit miffed at FFG on this. Now fans have to buy the same miniatures all over again.

Why couldn't they just come up with a set of skirmish battle rules and cards as an expansion instead?

Well you'd need new rules, cards, dice, terrain, movement rulers, and tokens. All that, just to have a much more limited market, as few if any who weren't already into IA would bother, as it would be perceived as an IA expansion/variant rather than a new game.

Seems like a terrible idea, that's probably why they didn't do that.

If you're just playing casually at your local game shop or dining room table, do whatever you want.

If you're going to play in official sanctioned FFG tournaments, follow their rules.