Sash window design / security - parting bead vulnerable to attack?
Being in the security trade, I can tell you that burglars are smart. It doesnt really matter if they know about that particular shortcoming or not. The main element to theft is speed. It will simply take too long to fiddle around with the bead strip in the hope to neatly displace the window. If someone wanted to get into your house a crowbar and a good kick will do the job just as well! 

With that said.. its your job to make as many aspects of your home as secure as possible. The sash security parting bead is probably the best thing you can do if you dont like the bolts, try fixing it with one way screws


Another option is to drill a metal pin into the bottom portion of the bead, but you may end up doing some damage. Or you could get some brass or stainless screws drill pilot holes into the bottom of the beading, set your screw till the head is almost in, then get a hacksaw and remove the head of the screw, then polish with a file.. 
that way, if they remove the beading there will still be a pin to make life more difficult.