.... Nuke em.

Alright, lets address something that hardly anyone even considers... the manticore missile launcher and deathstrike ICBM. Alright lets dispel some bullshit. Deathstrike missile launcher can have a 12 inch blast, why? because its not d3+3 inch diameter, its d3+3 inches radius. that 8 to 12 inch blast is str 10 ap 1 and denies coversaves and no center of blast. It also has a 960/unlimited range
(it gives two different values I'm going to say unlimited because its a fucking ICBM). Now the manticore does not have 4 blast templates, it has 4 shots that each give off d3 blasts. So in theory you could have 12 str 10 ap 4 large blasts in one game!

Now that we have that cleared up, lets talk about the downsides. First of all, the deathstrike has a possibility of never firing. Thats never good, especially for a 160 point unit. But thenagain, if it does fire, whatever its aiming at, is probably going to die. If fighting space marines, you could wipe them off the board, landraiders, terminators, tac squads all wiped away from the table. However, by the time it fires, (maybe turn two. Maybe never) it could be too late, meaning the terminators, landraiders, tac squads and such have already done their damage and are now too scattered across the board for you to have a hope of doing anything worth while. but again if you reached that point 160 points would really have made too awful much of a difference, not to mention its something that probably drew his fire away at least a little.

On the opposite end of the spectrum, from one megablast we go to a dozen smaller but still very lethal ones. The manticore is your answer to hordes. Absolutely devastating against large numbers of anything short of a space marine. At it's worst it will get 4 shots, at str 10 large blast with high range and everything, so it's about as good as a basilisk at worst. At best you get the ever devastating 12 large blasts. Now when you use this, keep in mind that you will probably only get 4 shots out of a basalisk or any other artillery/missile launching piece, before it gets blown away or becomes useless in some other manner.

So against hordes are there better options than the manticore? yeah.
Against space marine armies, or tank spam armies, are there better options than the deathstrike? yeah.

so why take em? who doesn't love them some bigass explosions? yeah, that's right, you know you do! and come on, it adds suspense and wonder and a whole lot of surprisingly orky fun to an army that is known as a pretty "safe bet" kind of army.