Don't know what you are trying but I know one of the reasons they want to use artificial turf is due to the weather and the field getting frozen which is dangerous.
I can assume you are looking for a reaction from me by posting that picture which makes it shiit stirring and totally of topic. Which to my mind equals trolling?
Sorry, but your smart-alec comment about making it
"look good on Wikipedia" was an unnecessary add-on that made the whole post look like you were just baiting.
If what you are really asking is
"why they would want to put an artificial pitch in Millennium Stadium when they have a roof and under soil heating" then why not just ask that without the added snide remarks? You'll find you will get better responses if you do that.
The reason they want to go artificial is because growing grass inside the stadium is very, very difficult without adequate sunlight and circulating air. Cardiff is at 51½° north latitude, the sun altitude never gets much above 15° in mid winter. Even with the roof open, the pitch area is in constant shadow, and the overall light level is too low. This is results in them having to grow the pitch outside, and install it by placing pallets in position and covering the area with soil before rolling out a new grass cover. This has to be done regularly (every year, or every two years?). Not only is this a huge expenses, it also results the the turf breaking up when it gets a bit ragged as the grass starts to die off.
There are other problems too. Growing the turf in less than ideal conditions makes it vulnerable to diseases and insect infestations. For example, in the past it has been contaminated by a Bibionidae fly (a.k.a. March Fly) infestation, which causes damage to root growth and surface grass coverage, and it has also suffered from Root Knot Nematodes, parasitic condition found in soil which attacks grasses and crops and can develop under certain climatic conditions.
If you look at this Google Earth photo I posted earlier...
....you will see the inside of the stadium is brown (compare that with the Arms Park next door where the turf is green) This is because at the time that photo was taken, there was no turf in the stadium, although it looks like the pallets have been laid and the soil is down. I would guess that photo was taken around June or July or 2009.