Interesting thread. I think I was about 8-9 years into studying piano playing when the topic came up (I was either in jr. high or high school. can't remember). at the time I was using Yamaha U3 at home, but my teacher had larger grands that I had access to if I needed it. the main issue was regarding my ability to control the dynamic range of the piece, and fatigue when playing LOOOONG pieces on larger grands. ppp and fff became more and more difficult with playing the upright. Part of the issue was because the action on my U3 was bit too light and was harder to control the dynamics, and also when I played a larger grand, I started to fatigue due to lack of finger strength/endurance, and had trouble towards the end of the piece to play well.
Our family didn't have the space nor the $$ for a grand, so my parents had the piano tech do something on the U3 to mimic more of the long hammer/key "throw weight" (I don't know the technical terms for this.. sorry) of the larger grands. I have no clue what the technician did. All I know that the action became heavier, which allwed me to build my stamina and strength, as well as give more control with the ppp.
Once I got the strength in my fingers (so it won't collapse when I use my weight to get the fff and sf) and ability to move fast with the heavier action, it became less of an issue when I moved back/forth between the upright and the grand.
In fact, once I mastered the slower upright actions, playing the grand well became easier, and truly a joy to play the nice grands. If you can play a fast repeat well on the upright, then when you move to play the grand it just played better, and you can concentrate on the expressions, beyond just playing the notes.
So it was always a pleasure to play on a grand (and I drooled over the semi-concert Yamaha grand my teacher had.. it had ivory keys, and I LOVED it. sweat slippage was a major problem with our upright that had the typical laquered key surface), but for practicing, it wasn't much of an issue that I had an upright. But usually few weeks before a competition/concour, my teacher would ask me to play on her piano more often leading up to the event (she had a steinway grand and a yamaha semi-concert) so I will learn to take advantage of the full range of the grand's faster action. So for me, I just needed the "final polish" on the grand, but everything leading up to it was on the upright, and it was fine.
I also had a friend, who was far more advanced pianist than me (same age), and she had a console at her house that she practiced on as long as she had been taking lessons. And she was a phenomenal pianist, with a scholarship to a well-known conservatory. Since she was such a great pianist, I assumed she had some awesome piano at home. but nope. Imagine my shock to find out that she was on a dinky little console, not even a studio!!! (she did do what I did, which is to start using the grand pianos at the conservatory leading up to an event). That sort of told me it's not the piano that makes a pianist, but the technique learned and the diligence of learning the correct keyboard attack methodologies (my teacher called it "attack". not sure if that is the conventional term used to describe how you actually depress the keys to get different tonal qualities of a note) were more important.
That and the passion to play, of course... :-)
and those techniques learned on an upright easily transfers between different pianos. And if anything, if you are able to control the tone on a "lesser" piano, playing on a better piano was a true joy, and allowed me to express better.
If I had kept the piano study much longer, I may have needed to move to grand piano for daily use. But it wasn't like I was vying to becoming a concert pianist, and once I went to college, just couldn't keep it up. soooo...
So in my mind, being able to practice on a grand on a daily basis is very nice, but not an absolute necessity, as long as you have some access to a grand somewhere (school, teacher, church, etc), if you're a very serious, advanced player. If you can afford it, it's great and ideal and you're lucky pianost! but not having one at home, I don't think it's that horrible of a penalty, like it would with you trying to do the same with a digital (digital, there's definitely a ceiling you will hit before it will start to hinder your progress.. and that level is at around advance beginner or beginning intermediate skill level).
Perhaps at some truly, far advanced level that I never achieved, you need a grand piano to excel. But since I never got there, I don't know if such requirement/need exists.. again, as long as you have some access to a grand to fine tune your performance.
Just a single perspective of a "once long ago a pretty serious student". not sure if it helps.
PS. this doesn't mean that I don't want one, and for those who can afford it, by all means they should get one, and I envy them.. I would LOVE to have a 6ft grand. just not able to due to space, and finance consideration
In the mean time I'm happy with my K8
and to upgrade from that, it will have to be a very very nice grand, I think.. very much out of my price range (and will have to either remodel the house, or buy and move to another one!)
...... 
