Doing a bit of research, I've found that in the current ruleset, there are actually no rules for the chainsaw that comes with the Sentinel box. It's for looks. Sentinels as walkers have stats for conducting melee combat and assaulting, like infantry can. Attaching the chainsaw doesn't affect their stats in any way though, and is just for looks. Ages ago, apparantly, it was introduced for the Catachan jungle fighters, and there were some rules about increasing weapon skill by 1, or adding an extra attack in melee, but that didn't come through to the latest rule set.
Anyway, I've seen people stick them on just because they like the look, and I thought I'd blutack one onto my sentinel to see if it would look more mech-like with two 'arms' - one a gun, one a chainsaw. I haven't painted the chainsaw, just stuck it there to see what it looks like.
If anyone has any opinions on the look of 'two-armed' war machine vs the normal 'one-armed' look, any comments are welcome, as I am undecided about whether I will stick chainsaws on any of my figures or not, since they don't affect gameplay in any way and are just for looks.