Lewis vs Aus 5 wkts @ 18.60
Jones vs Aus 3 wkts @ 51.66
Compare apples with apples, please and don't include another form of the game for one but not the other.
In ODI's, given it's only been one game, but Jon Lewis got pummelled for 69 runs off his 10 overs and wasn't picked for the rest of the series. Jones got picked eventually and largely bowled well even disregarding his stats. Anyone who watched him would have seen the Aussies, save one game, found him more difficult than not.
As I said mediocre spinners very rarely have an impact at Lords. It is surely more beneficial to substantially improve your batting than to pick an extra bowler who will have no impact
He averages 1 run more per wicket than his overall career average and falling rapidly. Additionally, he took 9 wickets in the last match he played there (2004) against a team including such 'mediocre' players as Brian Lara, Shiv Chanderpaul and Ramnaresh Sarwan on a belter of a deck. If it wasn't for him, in fact, the game would have been a dull draw with 5 tons, including one double ton, scored in it. Giles won the game for England.
http://www.cricinfo.com/db/ARCHIVE/2004/WI_IN_ENG/SCORECARDS/WI_ENG_T1_22-26JUL2004.html
Except in terms of depth, my bowling lineup is no worse than the one selected, and the extra value added by Pietersen will far outweigh the innocuicy of Giles.
Giles isn't innocuous and, as shown above, recent form suggests he is well aware of how to bowl at Lords. I saw a lot of that match and he bowled brilliantly.
Giles has a perfectly respectable job to do (tie up an end whilst the quicks do their job or bowl to take wickets if the ball is turning) and does it well. And in his last 10 Tests, he's been far from innocuous (39 wickets @ 27). And if you include Tests further back, his average is still below his overall career average.
And anyway, it's not just his individual ability; it's how he fits into the TEAM. There are probably more naturally talented spinners in England but I doubt any of them would click as well in the team as Giles has. With him coming in behind Harmi, Hoggard and Jones, that's a potent attack. Hoggard also bowls very well when no runs are being scored at the other end which, again, is Giles' speciality. Take Giles out of the team, and the bowling attack as a whole loses more than just their spinner (he's no mug with the bat, either). The above combination is what has won England Tests in the last two years. There is no good reason to change it.