I think it's a spectrum, one end very extroverted, other end very introverted and everyone else in between - some of us hover in the middle (I tend to introversion but only moderately according to MBTI test).

From my own perspective as a moderately introverted person I will say I can find people who are extremely introverted quite difficult to read, they don't usually give out much to go on and I can find conversations harder work (at the same time I'm aware that I prefer others to do the talking myself!). That said I do know the difference between rudeness/aloofness and introversion, because of my own tendencies. There's no way I would discount someone as a friend because of that one trait even if they were very introverted, I agree with S4nd that it is just one trait and there are so many facets to a person other than that one aspect.

Also would just like to add that there is a difference between shyness and introversion, it's even possible to be extroverted and shy. I personally am not particularly shy but am still introverted in nature.

I don't think it's necessary to socialise only with others on a similar point of the introversion/extroversion spectrum personally, surely variety is good in friendships?

The only problem is if as in LucyG's experience people do not respect others wishes, but that seems to me to be more of a boundary issue.