Quote from brainfade (who is anti-EDI as well as anti-BRS)...
"I put in the bit about 'G2' for because the EZY we were waiting for (to land) looked to be so far out that perhaps we could have been allowed to go. Believe me, at CDG, we would have been away!"
Tower controllers judge gaps every single working day, in many cases several hundred times every working day. Guess what? Most of them are pretty good at it. Often, a gap that looks usable to a pilot isn't, sometimes gue to simple visual perspective and sometimes because there's a reason that can't be seen from the cockpit. That could be slower departed traffic ahead, overflights crossing the departure path, too much traffic on the sector to which departures will be handed over, the list goes on.
Most tower controllers derive great satifaction from firing off departures as quickly as safely possible, and in the smallest usable gaps between arrivals (I certainly did!) so calling when you're at the hold, and can be seen perfectly well by TWR who has a vested interest in getting you airborne and off his frequency ASAP, is utterly superfluous.