Big companies like EM need to have a flexible program to allow employees to get company wide experience and that includes expat assignments. I was lucky enough to have 2 in my em career, one for 18 months and one for 5 years (both to the US). Both of mine were for quite specific reasons and had an end goal with respect to benefit to the company as well as a development opportunity for myself. As a UK based employee, I was happy with my expat deal, which on balance gave me a final package that made me equivalent to a comparible US employee in terms of total remuneration whilst on assignment. i.e. this was a reasonable cost to the company. What I have seen is a large number of US expats working in the UK, Asia and Europe, which is a much more expensive exercise. US employees typically earn 1.5-2x more than the UK, so when you add expat premiums and expenses on top of that....US expats are much more expensive. Additionally, you have your "career expats" who spend most, if not all of their careers on expat assignments. This was especially true in Singapore and I never understood how this was justified or tolerated. One US guy was there for 20 years on an expat assignment! Projects were pretty guilty of this practice too.
So yes it is abused, and like most other things can spoil it for those who genuinely benefit from an opportunity to go on an expat assignment. It is one of the reasons people join a large multi-national so I don't think you can complain about it in principle. When abused though, it can be demoralizing for other employees, especially when the opportunities seem to be restricted more to the "club" of top rankers!