On its own, this cape is not anything to write home about ... maybe a quick email is what it's worth, EXCEPT to Blm and Smn, both of which receive a level 50 Ring (endgame piece) that needs 75-76% HP (some SE glitches on exact percent) and < 100% TP to be activated. The Convert HP > MP (as Grey Messenger explained elsewhere in this thread) on this cape makes it valuable in reducing the 25% HP. Oddly enough, WHM and BRD also have level 50 endgame rings that are similar in power, etc. which will activate only when HP is yellow; the convert HP > MP feature does not work for them.
Hmm, anyone ever noticed a mis-type on the "Converts 20HP to MP" part, on all other items theres a space between the number and the HP/MP, see for yourself ^^
but "Converts 20HP to 20MP" has the same effect saying "HP -20 MP+20" therefore it would make sense to say "HP-20 MP+40" since the Blue Cape +1 does in fact add an extra 20 MP without the conversion thing and it is safe to assume that all jobs have more than 20 HP at lvl 68.
The only difference between "converts" and "+/-" is that converts will only change MP to HP if you have enough MP for it. So if you only have 5 MP total, a physical earring will only give you +5 HP and -5 MP. With this being said, I seriously doubt any Lv. 68+ would have less than 20 HP.
Actually... another difference is that latent gear, like sorcerer's ring, have a desirable latent effect that only activates when you have less then 76% of your current HP minus the effects of conversion gear.
So this cape, by converting instead of lowering your HP, doesn't lower your latent activation point... where as +MP/-HP gear does lower the latent activation point.
Both could be used to lower your HP in a macro, but the conversion gear doesn't need to be removed for you to gain the latent benefit. Indeed... most races could nuke in full zenith gear and be receiving the sorcerer's ring latent even if they have full HP.