I personally can account for a quarter of my match wins to lockdown victories- if it wasn't there, I think about half of those victories would've turned out as losses as our team's reinforcements were nearly out/completely out and we were hoping to hold just a minute longer for the lockdown victory.
In its current implementation, though, I'm not too big of a fan of it; specifically, if you've got twenty seconds before lockdown and your attacking team is in a contested cap zone, the lockdown timer should "freeze" at 0:00 until either A) the objective is capped whereupon the lockdown timer has more time added to it or B) the attacking team is killed/forced out of the cap zone, causing a lockdown victory for the defenders.
Also, perhaps a verbal and/or visual cue for both attackers/defenders when the lockdown timer is nearly at 0:00 (perhaps 1 minute before lockdown) would be useful. Furthermore, the verbal/visual cue could be used to inspire the attackers to push objectives before the timer kicks in (i.e. Inspirational comments by Commander (TL), etc.) and for the defenders comments about how they only need to hold their ground a little longer (i.e. same as above, though with a "Hold to the last man and the last round" style commentary).