The Saladin isn't exactly flawless, though. No armor on it's side or rear, skids a lot, and has to spend move points getting into a firing arc... and it costs 100 more BV.
If you're setting up a random "equal side" fight scenario, sure, make the BV comparison. But if you're playing a Merc ATB campaign, I think it's a different question, because the Saladin costs about 2/3 of an Urbanmech. It's also worth noting that the vanilla Urbie is an AC/10, not an AC/20; Liao is the one who drops some armor to make the upgrade, which also ups the cost.
The fact that the Saladin has three vehicle crew versus a single mechwarrior for the Urbie probably means the Saladin's personnel costs are a little higher, but it'll be cheaper to maintain than the Urbanmech. If I had 3 millon C-Bills lying around to spend, I'd definitely take three Saladins over 2 Urbanmechs of either variety. And if you told me I had to fight a battle in one of the two, I think I'd rather be in a speedy Saladin than a plodding Urbanmech.
Even going by BV, if I had to fight a battle with five Saladins or six Urbies, I think I still go with the Saladins.
I don't agree with your fundamental point though, Kanil; neither is the most effective use of BV. (The Saladin may be high in terms of C-Bill efficiency... I haven't thought about it in a really long time.)