You're your character. If you imagine him/her as a stealthy assassin then you don't want to run around in massive full plate armour. You'd most likely go for something light and simple. If you don't care then fine. This is actually a system you don't have to use. You could also ask team mates not to use wings/pets if you feel those things creates too much lag when progressing.
To some, design is important, to others functionality is enough. In fact, you could run around with a sticky figure fighting sticky monsters in a low graphic world. Items could just be stat boosters. No visual effect what so ever. No shiny stuff nothing. It would work and you'd most likely would not care. Maybe this would even make you happy if it reduced lag/what not which has a negative effect on progression. Which is fine.
Diablo 3 (and most other games) isn't only designed to be effective and performance wise super great. They're designed to look appealing and in those boundary's perform well.
I love transmorging. To me its equally important how the character looks as it is how it performs. That's the way I play. Luckily for you, cosmetics have nothing to do with progression and as said, you could just ignore it. It do have huge value for other people though.