There are three main problems with the spell:
1) What "appropriate special abilities for a creature of half HD" means is undefined; monsters as statted just have a bunch of abilities, they don't have a an ability progression to point to and declare that certain abilities come at certain HD. If efreet didn't get their wish-granting ability until they reached 10 HD, for instance, then making a 5-HD efreeti simulacrum wouldn't be nearly as abusable.
2) The HD = 2×CL cap. Summoning spells all have a cap of around "your CL in HD when you first get the spell" and don't scale (except gate, but that has its own problems). If planar binding can call a 12-HD creature whether you're CL 11 or CL 20, then simulacrum shouldn't be able to create a 20-HD copy of a 40-HD creature when you're CL 20.
3) The inexpensive monster part component. Rather than requiring a free and ignorable monster part and expensive but generic ruby dust, either the expensive component should incorporate the monster part (an emulsion of the monster part in an expensive alchemical mixture, say) or the monster part should be the target of the spell rather than a material component, so you can't arbitrarily mimic any monster you know.
Point (1) can only really be fixed if you overhaul 3e monster creation to the point that you have a rigorous idea of exactly what a "7-HD gelugon" or "11-HD solar" or whatever looks like ability-wise, but (2) and (3) are easy enough to fix.
The larger problem, of course, is that once you fix those abuses simulacrum isn't really worth casting; an utterly-controlled minion of middling HD is nice to have, but not when it costs you hundreds of XP and gp to create and thousands of gp to heal and it's cheaper to just call or dominate something instead. Even in the case it's most likely thematically intended for, making a copy of someone to use to replace them, you have to exercise control in person and the spell doesn't say the simulacrum has all of the original's memories so it's somewhat of a shoddy duplicate for anything sensitive.
It might make the most sense to split the spell up into two separate versions, one "permanent combat-capable minion of a generic monster of my choice" version (which is much less expensive, has no mind of its own, and can more easily be healed) and one "deep-cover clone of a specific individual" version (which is still expensive, requires the original to be be under your control already, and can be controlled from a distance).