Grinding it changes the structure and makes the starch (sugar) much more easily available. It has less fibre in that form than whole rice or rice cakes. It also does nothing to encourage gross motor skills or speech.
No, grinding something up does not make fiber disappear. If it did, then wholemeal flour would also have no fiber in it--it's ground-up wheat
As for arsenic, all rice contains similar levels of arsenic, and there is nothing special about baby rice in this respect. If you give your baby rice in any form, you are giving them a few parts per billion of arsenic (shrug). I live in a country which eats a LOT of rice and has the world's longest life expectancy so I am not too bothered about this!
What do you mean by "whole rice," by the way? Are you referring to brown rice? Brown rice of course contains more fiber than white rice, but you can give brown rice in any form, including brown rice infant cereals if you want. There are brown rice ones available.
Hate to tell you this, though, but brown rice contains more arsenic than white rice, regardless of whether it's baby rice, rice cakes or boiled rice served with a curry.
Mind you, it's not clear to me why we are supposed to be worried about "fiber content" in something that babies are consuming in such minuscule quantities anyway. It's not like this is going to be the mainstay of a child's diet for years on end--they are having a few teaspoons a day for a month or two!
As for "gross motor skills or speech".... the same could be said of any soft food fed on a spoon, whether it's rice, oatmeal, yogurt, purees, soup or anything else. It's important that children get lumps and texture as well for a whole variety of reasons, but you can give a child baby rice/infant cereal AND give them lumps and textures! My baby has had all kinds of food ranging from baby rice to lamb chops gnawed off the bone. It's all good.