The question becomes, will the lowered theft rates compensate for the lower sales from people who don't think it worthwhile to hunt down an employee for a toy. Toys are often impulse buys, something you (or your kid) grabs and throws in the cart because, eh, why not, this looks like some harmless fun. Having to get someone to get it for you takes the impulse out of it.
Not to mention the annoyance of toys shipped in assortments, which the customer can't look through to find the ones they want. If I see all the pegs full of first wave Transformers while I'm looking for the third or fourth, is it worth the effort of getting the case opened, or am I just gonna shrug and move on? (Let's not even go into the Hot Wheels Guys who are there every day.....)