As of 2006 (when I left the legal tech world), the transition to Word was already well underway. Law firms hated it, but like everywhere else it was becoming The Way. And before Word, there were no "obscure" versions of WordPerfect; it was very widely used, in and out of the legal industry.

Everyone loved WordPerfect because of its ability to tune the final output. Formatting requirements for some court filings are very specific. Back in the day, it was like "12 point Courier, double spaced, no more than 50 pages in length" for appellate briefs (that's from memory, may not match the actual situation at the time) and it was important that an electronically transmitted document look alike on every machine, something that's virtually impossible to guarantee with Word.

This was in the day before print-to-PDF drivers were available, and no one was going to buy Adobe Acrobat licenses for every attorney, paralegal, secretary and word processor in the firm. Wordperfect was well-loved for a reason.