Neither of the supplements are essential for Gloranthan gaming, though they're clearly nice-to-have. A large proportion of the Bestiary is critters from Pamaltela, which means either you don't use them unless your game goes to Pamaltela, or you fudge and use the critters in unusual circumstances in Genertela. Nice to see stats for Slarges, though!
The rest of the critters were initially valuable for their RQ3 writeups, as prior to the Bestiary many monsters had only existed with RQ2 stats and had to be converted by GMs. Now that's a moot point - if you have RQ2, MRQ Monsters, etc, you may find you already have lots of the critters already available. Personally, I'd get it (price depending, of course) - just for Gloranthan critter completeness.
Elder Secrets is remarkable for the appalling artwork
and some really good content. The Secrets book itself contains some excellent essays, though these days you might find a lot of it published elsewhere (GSA, the HQ Glorantha Book, etc). The Elder Races book again contains writeups of Trolls, Elves, Dragonewts, and some cults, etc, which you might find elsewhere, although there are some tidbits which you won't find anywhere else. It's certainly useful for a writeup of how Gloranthan metals and crystals (especially iron) *should* work. I'd grab it, again price depending.
Incidentally, have you looked at some of the HQ books, in particular Anaxial's Roster and Introduction to Glorantha? You'd have to more or less write your own stats for the monsters in Anaxials, but it's v. good Gloranthan info.
Cheers,
Sarah