You're right, for some reason I just assumed all ERP's are as easily extendible as Microsoft's. It just makes sense they would be oriented that way due to the complexity of the world. I often hear horror stories about SAP, and can't for the life of me figure out why it's so popular(except it's more oriented than other ERP's to non-tech savvy people).
I work in this industry(though thankfully very rarely directly with SAP) and I cannot fathon why it is popular either. As I explained in my other answer in this thread, everything they touch turns to garbage.
I think it's really a case of a self-fulfilling prophecy, of a sort. They are the biggest in the business, so people go with them, which makes them bigger, which attracts people.. and so on. It probably helps(SAP) that they also decades worth of man-hours implementing all sorts of insane business logic(like for accounting, as you mentioned, which is something you definitely don't want to get wrong) where other players are likely to be behind.
If you dig you find as much screwed up ERP projects for SAP as you find for Microsoft or for any other system. Differences in absolute numbers are caused different market share of the systems in question.
