Say, what is the first thing that comes to mind when you talk about Tome of Battle? Most likely, the maneuvers - the thing that made the close-quarter combat fun for even those players who don't like to just smash things with a metal stick 'till it breaks. These maneuvers are so awesome that a 1-level dip or the Martial Study feat are really good options just to cherry-pick some stuff to give a non-ToB PC another level of badassery. They are just smoking burning rad, and that's the fact.
Then, we take a look at PrCs. Some of them are weird in fluffy part, some of them need you to be, let's say, an elf... And some of them make you drop the latter half of your maneuver progression while giving you mediocre to weak character options (yeah, I'm talking about that weird Shadow Sun Ninja). Pretty much the only PrC that doesn't require your character to be a battle psycho, a pointy-eared or heavy-bearded guy, or to multiclass/give up on maneuvers is the Master of Nine.
But looking at the PrC Tiers list it looks like the Master of Nine is the worst ToB PrC to take (-1 Tier, which is hard), while weird, not very useful and easy-to-almost-fully-replace-with-a-"+1LA"-Template Shadow Sun Ninja gets a "+1 Tier" for... I don't know what.
As far as I understand, the Maneuvers are the most valuable resource in ToB (the second one are Stances, which are nearly as cool), and high-level maneuvers are nearly unobtainable without being a dedicated martial adept. Getting not-too-hard-to-resist abilities of being a light show at-will? I don't know, but even getting an ability "to burn a small village or an army every 12 seconds" sounds better to me, not to mention ability "to pick maneuvers and stances from ANY of the Nine Schools" which Master of Nine gets. But maybe, I don't understand something?
So, here's the question: Why is it a bad idea to get the Master of Nine PrC? Is it really not worth those admitably not-very-nice (Dodge? Yeeah, riight...) prereqs? What makes it not worth the efforts?