I mean he got the girls to the clinic, make them believe they were going to be cured,then brainwached them in order to got them spray the virus in their countries in the future, via a hidden radio.
Hum...wasn't it simplier to just send a dozen of henchmen away to spray it ?
Henchmen can talk, or be traced easier. The idea of the girls that they would be pawns and because they were brainwashed they wouldn't even know they were carrying out his orders. If you didn't know the common thread of them being at the clinic it would be very difficult track Blofeld down.
It's also a common spy gimmick in movies like Telefon, or in The Americans (TV Series). Modern terrorists use more blunt force methods, because they don't want money, titles etc. They want death.
Blofeld wanted to get his title (Why he didn't want money as well, made me wonder, maybe running SPECTRE already made him rich, maybe he just wanted legitimacy).
I guess its more sinister to have Blofeld mind controlling these sleeper agents. It also gave them a reason to have a whole bunch of girls hanging out around Bond for half the movie.
Blofeld's schemes rarely made much sense, but thats part of the fun of 007 movies.
If those girls were ever picked up they wouldn’t be able to talk as they didn’t even know what was going on since they were hypnotized , henchmen could rat blofeld out if enough pressure was put on the,.