Why? Because, if you take the time to read or at least peruse their 330 page Introduction to their NT (amazon.com) (published in a second volume with an appendix), you see that they fill in most if not all of the details of the how & why of their edition.
What sort of stuff? Well ... who'd've thunk that two pages on casing of κυριος, Χριστος and υψιστος would be appropriate? Sometimes on reading an upper-cased Κυριος or a lower-cased χριστος (In NA27, not WH) I've often wondered "why is that one upper-cased/lower-cased?"
WH show their work and tell you why, at least in their edition — and why, in four instances in Luke (Lu 1.32; Lu 1.35; Lu 1.75; Lu 6.35) they also upper-cased Υψιστος. They take two pages (pp. 316-318; §§414-416) to tell you. Here's §416, explaining their capitalization of Υψιστος:
416. An initial capital has likewise been used for Υψιστος in the four places, all in St Luke's Gospel, in which it stands in the singular without an article. In this shape it exactly represents the anarthrous Elion, a very ancient name not confined to the Jews, and is virtually itself a proper name. In the LXX the article is usually inserted: but in Ecclesiasticus, doubtless a better authority for Palestinian custom, Υψιστος occurs frequently, and has the article but once, except in combination with another title.
More than you ever wanted to know, but helpful nonetheless. If you're looking for a copy of WH's Intro/Appendix, then you want the 1896 edition which has corrections/updates.
Why do I like this so? Whether I agree or not, I can at least know what in the world they were thinking. You can't do that with any other print edition; none are nearly as transparent as WH were. We'd all do well to re-learn this lesson.
Wipf & Stock have done a recent photo-reprint, available in paperback (amazon.com).
Hendrickson did a reprint in the late 1980's, in hardcover (amazon.com). Some used copies of this are available via Amazon.
Which do I recommend? I don't know, I've not used any of the reprint editions. Years ago, I found a copy of the 1896 edition via abebooks from a seller in Australia and snapped it up quickly.
If you work with the Greek New Testament and do anything remotely pertaining to textual criticism (the appendix "Notes on Select Readings" is a mini-Metzger for WH's edition and their "Notes on Orthography" will tell you more than you wanted to know about spelling in their edition); or if you have interest in orthography, punctuation, and other particulars of producing and fully utilizing a printed edition of the Greek NT, then you need this book; whether the Hendrickson hardcover (amazon.com) or the Wipf & Stock softcover (amazon.com).
Update (2008-05-14): Thanks to Mark from the Bible and Tech blog for pointing out that WH's Intro/Appendix volume is available via Google Books. So grab it and absorb!