It's just a waste of time. I know some Summoners do it to have Titan grab one mob off of the tank during dungeons, to lessen damage the tank is receiving, but it very rarely compensates for the dive in DPS the Summoner is inflicting on themselves by not having Garuda out.

If the tank is getting bodied, or needs help with hate, at that point the Titan idea can be pitched. But if you go in with it, and refuse to even give the tank a chance, then yeah, they'll probably just leave. Tanks get instant queues pretty much across the board, and most are well aware of this fact. You may find it a bit prima donna, but it is what it is.