Simply put - why not? Yes we technically have a 2nd lefty already making the bullpen in Hisanori Takahashi, but I view him more as Darren Oliver-type rather than a shutdown lefty-on-lefty reliever. Here are Shouse's splits from recent years:
2007 vs LH: 23.2 IP, 7.99 K/9, 2.28 BB/9, .212 AVG, 3.26 xFIP
2008 vs LH: 29.0 IP, 8.69 K/9, 0.62 BB/9, .180 AVG, 2.11 xFP
2009 vs LH: 17.2 IP, 7.13 K/9, 0.51 BB/9, .231 AVG, 2.55 xFIP
Or a much easier way to put this -- 3 year splits vs LH: .203/.228/.303 -- also known as: domination.
In order to fit Shouse, the Mets would likely need to option both Parnell and Green to AAA (and Mejia to AA), but certainly none of these are bad things, in my opinion at least.