I don't agree with that. There is much, much more to Eberron than Sharn. In fact, I'm annoyed that every adventure published either Starts in Sharn or Visits Sharn. Eberron is more than Sharn. Even if your adventuring group has a layover in Sharn, it doesn't necessitate the purchase of the book. Xen'Drik is only important if you make it important, since it's a long travel away and removed from everything.
I don't think it's a radical statement to say if you aren't making Sharn or Xen'Drik pivotal locations in your campaign, you don't need a sourcebook on them.
A central theme in Eberron's background. The likelihood that the PCs encounter a dragon is very slim. Even if it's a dragon in disguise, well that's not really helpful.
I'm going to agree with you on this. As much as I love the Sharn book, it isn't required. I did my first Eberron campaign in Sharn and loved it, but my past two haven't touched it at all and have had very different feelings.
The second was in the Lhazaar Principalities dealing with pirates, sea battles, slavery...good fun stuff.
This current one is with the entire party playing as goblinoids in Darguun battling to see that their Dhakaani clan rises to the top in the reborn Dhakaani Empire.
Both are fun, both have nothing to do with Sharn or Xen'drik.