I was first enraged by this topic a few weeks ago on Reddit. I cannot find the post now, but it was about 'Using Forge World in games'. The original question was along the lines of 'do you use them?' Posters hemmed and hawed about it, and then a few came in with very hard-line views: if I don't have that 80 dollar book, I don't want to play against that person using those rules. I can't know how to fight them!
This could be a valid response, if you owned all the regular codexes. If you're willing to purchase all the regular codexes and memorize them, though, you probably have the time and cash to purchase the Imperial Armour ones too.
There is also the idea that you could play someone to learn how the units work on the field. Throughout history generals have had to adapt during the battle and change how they're fighting. You can too! I realize, probably more than a lot of people, that your time is precious. I only get about three hours a week to get in a game of 40k, Warmachine, or Fantasy. I don't want to have those three hours ruined because someone used something that beat me because I didn't know what it does.
The thing is - you can learn on the fly! Be a good general. Plus, think about that other guy! He just spent a vast sum on this super sweet resin model and you want to take your ball and go home because you don't know every super secret netlist strategy for it. What is he gonna do now?
That unit is B-R-O-K-E-N!
Many people, seemingly the guys that never play and just TALK about games the most, seem to think that Imperial Armour rules are broken, never tested within the bounds of the game, etc etc. I don't know the actual data but I'd suggest that most of these dudes have never even touched an IA book.
If you read most of the entries, I'd say 95% of them are overpriced. The land raider variants are huge points. Even the Contemptor, the dread we devoted March to, is very pricey on points (not to mention the wallet). It is very safe to say that almost anything you get from IA is not going to earn back its points, a measure that many wargamers use.
There are a few examples of rules that were not thought out well. The dreadnought drop pod used to be really, really great (and still is), the lifta-droppa had no faults, I've heard the driller is super nasty. The thing is! If somebody is using those against you and wiping the floor with you, ask the dude to house rule it. Make it more points, add something to make it fail, etc. 40k is a social contract game - the street runs both ways. It is better to house rule something so everyone has fun than to just blatantly write off things because you can't afford them or won't purchase them.
Different ways to play!
I would hazard a guess that I've played 40k at least 75 times in the last year. Do you know how old it gets playing the same three missions, same three deployments, against the same old lists? IA units add variety to the tabletop; something really cool that catches your eye and changes how you play the game. Contemptor dreads are not just boxes on pegs. They look awesome and have a variety of weapon types that you normally wouldn't see.
Tyranids have a ton of different options as well. Hell, formations change the game so much. Flyers add a ton of new rules to the game that open up new strategies.
In the end, the choice is yours if you want to make 40k serious business. Just realize that Games Workshop is trending toward Forge World - not away from it. The most recent book with the official 40k stamps are proof of this; the Chaos Dwarf official list in Tamurkhan is proof of this. 6th Edition will likely embrace Forge World, and then your serious business game will look pretty shoddy.