Jack of all trades
Is quite an old concept. Your "T" developers will typically not be as good in one of their "deep" skill sets area than a dedicated specialist in that particular area (unless it is a very small area), purely due to the time taken to fully master an area (especially when things are ever changing and so an area must frequently be revisited to keep up to date).
Specialists can be useful. e.g. I'm sure TalkTalk could have done with more PCI-DSS & pen-tests specialists