In my opinion it's not safe to place total reliance on TCAS in class G airspace, and any pilot who does is negligently risking the safety of his own aircraft and other aircraft which (in the UK) are not required to carry a transponder.
Chevvron,
I assume you mean "Fly in VMC whilst heads in and not bothering to look out"?
I doubt very much that anyone does that.
On the other hand, if you mean being obliged to fly IMC in Class G with no ATC radar service - unfortunately there is increasingly no choice in UK's Class G airspace. Progress has been backwards in that respect; thirty years ago one had a choice of radar units - these days one often struggles to obtain a Traffic Service for the entire journey.
