to be honest, i think to the untrained eye (ie most of us), many of the species will look pretty much the same as some of the others. do you really want every single sphodromantis species for example? how about hierodula? so really having every single species wouldnt be as great as you maybe think. but it is a dream after all.
maybe better is every genus, but again, many genus look very similar as well. in fact, most mantids look similar and compared with the wild and exotic ones, are pretty boring looking.
Will be lucky to just see 10% of all the mantid species in any insect zoo/collection center, especially when the USA only have probably about 1% of all mantis spcies in the world.