Haha, funny. I didn't read all the responses, but I guess I did it the wrong way for my last piece (a woodwind quintet) - I gave myself (the conductor) a concert score, and my performers transposed parts, and then just transposed in my head when I needed to reference certain notes. But, there are a lot of reasons for that.
When I took my score into lessons every week, generally my teacher would be playing piano, so it's just easier for him to play five voices when they're all the same transposition, and the point of my lesson isn't to test my comp teacher's transposing skills (which were fine - I accidentally brought in a transposed score once and he played it just as well xD) but to deal with problems in my composition, so that's the bigger issue. He did make it a point to remind me to note whether it was a concert or transposed score, though.
Then also, for myself personally, I was rehearsing my piece with people missing every so often, and it's a lot easier for me to sing the missing parts (voice is my primary instrument, so that's all I could really do) when it was in concert pitch, so that's just a random consideration. It depends on what kind of singer you are, I guess - some singers have no problem transposing while singing, but I'm not really one of them, because I tune based on the sonorities and it's hard for me to sing a pitch that doesn't appear to be in the chord from first glance.... and my transposing is terrible as it is, being a singer and never having to deal with that before like, this year.
I definitely don't have problems reading transposing scores when called for, but I am not a conductor, and as a composer I find it a lot easier to work with a concert score unless I am just trying to give myself practice. It's good to look at the transposing version for range checks and the like, but typically it doesn't seem necessary to me until nearing the end of the compositional practice, especially if you just keep in mind the range limitations in terms of sounding pitch. But that's just me... not really based on any facts, just personal experience, limited as it may be.