well, that's the thing, better to have and not need, right? if you know you're hitting that MCT facility tonight, i'd rather have a mana barrier or a healing preparation on me and have it fade away then to not have it when the HTR team comes calling. there's also the fixation metamagic to consider, which seems like a requirement for any serious alchemist. it's a bit more expensive than it really should be in terms of karma, i admit, especially compared to quickening. it does seem like alchemy is more of an NPC thing, though i did have a decent ninja alchemist mystic adept ginned up. those shuriken with a force 8 punch on them hurt like a bastard.
despite the cost and duration issues, if there's no real other way for a mundane to have magical abilities then you kind of have to take what you can get, no? i can see a lot of people who might reasonably expect to be hurt and/or need a mana barrier and/or want to turn invisible in the next few hours other than shadowrunners. bodyguards, HTR responders, VIPs leaving their arcologies for any reason at all, Johnsons expecting trouble at a meet, etc. several detection spells are the sort of thing you'd want to have just a bit more of an edge in, well, a lot of situations. detectives, pick up artists, suspicious wives, hunters...
for that matter, isn't it cheaper to have the DocWagon mage at HQ dial up some healing preparations to hand out to the trauma team then to have him risk his ass on a hot pickup? better yet, he can work his healing magic at multiple incidents this way, rather than having to actually be present to work on patients. same situation for an HTR team; i'd rather have a team alchemist gin up a few heals and mana barriers every night and have them ready to go on the sortie instead of having to have the very valuable mage directly exposed to fire. especially if said alchemist is a second-stringer or aspected alchemist that would probably just get killed in a real firefight anyway.
as far as legality, i can't imagine a preparation for a healing, detection or protection spell would be illegal very often as long as the maker is licensed. the point about flexible signature is well-taken if for no other reason than said alchemist wanting to avoid the red samurai knocking on his door trying to find out who bought from him earlier in the evening. control thoughts or combat spells, yeah, those are probably going to be illegal as all hell to possess. except, of course, for the brave boys on your friendly neighborhood HTR team.