Patent troll, no. Bad patent, yes. Ideas shouldn't be patentable. Implementations of ideas should be.
The defense of a patent should never be "They're doing something similar to what we're doing."
Even from the point of view of what was considered state-of-the-art when the patent was filed, it appears to be a logical (obvious) extension of then-current standards.
As such, it would seem that XML as a whole (or at least many implementations thereof) would infringe on i4i's patent. The patent is for the definition and storage of metatags. So basically, if you are using XML to store data, you cannot allow the end-user to define custom fields within the XML without paying royalties to i4i.
Of course, i4i could also find themselves in a sticky situation by suing the company that makes the software that their software depends on (nearly all of their products are add-ons for Office), and Microsoft is under no obligation to license software to i4i (businesses have the right to refuse service except in certain civil rights circumstances, and I don't believe that being hostile to the business is a protected exemption).