If taste is included in your list of benefits, I would say yes.
In all seriousness though, I'm not sure. I think there have been studies showing the addition of milk to diminish the health benefits of tea, but I don't know about sugar.
I would guess, though, that a sugar cube just adds simple carbohydrates to your diet, and doesn't negate any benefits of the tea itself. One sugar cube really isn't that
much sugar either, so I wouldn't worry about it.