For me now, it almost is a deal breaker. If I'm considering a particular light that doesn't tail stand, it better have some other features that make it really stand out.
I use tail standing all the time. Sure, you can lay it on it's side to hit a wall for bounce, but that light is very directional and casts heavy shadows.
When you bounce off a ceiling, it's exactly like having a ceiling light on.
With tail standing, 90% (my arbitrary guess) of the light you see in the room is what comes from the bounce of the bright center beam and then some from the spill.
Whenever I get up at night or early morning, I never like to turn on main lights because I'll wake up everyone else in the house (including the dogs). So I keep an EX10 by my bedside for bathroom excursions at night, but also for cranking up the level and tailstanding it in the kitchen to make coffee.
And of course, there's always the power outage scenario when your tailstanding LED lights can work as well as any LED lantern. For those times, what I do is to tail stand the light and put it under a table lamp so that the beam will light up the lamp shade as well as to hit the ceiling through the hole at the top of the lamp shade.
With the EX10 on only a couple of clicks below max (for better runtime), you'd think it was the regular lamp itself that was on.
Once you start actually using a light as a tail stander in the many situation where it's handy, I'm willing to bet that you'll start depending upon it more and more.
My most favorite "go to" tail standing lights are my EX10, 120P, D10, LF5XT, and JetBeam I MK IBC. (I sold my NDI mainly because it wouldn't tail stand.)
With just those five lights on their tails and set to mid-bright settings, I can pretty much light up the whole house all night long if I wanted.
I keep eyeing the Coast LED lanterns (both D and AA size) and keep thinking I should have one for power out situations, but I keep finding that a small, single cell light on it's tail is quite sufficient and without taking up as much space.