I disagree. I think BCSL is one of DT's most melodic albums, actually. Look at The Count of Tuscany and The Best of Times, let alone the rest of the album. Maybe the songwriting, yes, in terms of what I call the poppy side of Dream Theater, but there's still some of that, I think. At least in Wither, and I would argue that A Rite of Passage and the midsection of A Nightmare to Remember have some of those attributes.
DT12 is just way too heavy throughout to be representative of Dream Theater's overall sound, to me. Even Along for the Ride is a pretty heavy song. Plus, the album is entirely lacking songs in the 10-15 minute range, the mini-epics that are so important to DT's sound. It's short, generally more straightforward songs and then Illumination Theory. It's not really a well-rounded album in my judgement at all. BCSL is.
I think Mike Portnoy was right when he said that BCSL is an album with, as I recall, TGP, LTL and ACOS (plus maybe a couple others?) all on one album. It really is a summary of all of Dream Theater's sound, encompassing all its different aspects (which couldn't really have be said at the time about any album since Scenes from a Memory).