Zerglings into Roaches?

What? I don't understand what you mean. Zerglings CAN be morphed into Banelings. It isn't really suicide to use Banelings btw. It may appear so, but when your Baneling suicides (a.k.a reaches it's target) it does exactly what it's supposed to do. Even if it dies, it's a good thing. It's only bad if it dies before reaching it's target.

I like the Salvage ability, if that's what you're asking for. But it can't really be compared (or maybe I am just misunderstanding you. Care to elaborate? )

What if a Protoss goes heavy Zeal and you manage to tech up to Roaches? If you have Zerglings left, you'd want to morph them all into the currently more useful Roach. Zerglings arn't ALWAYS useful. Sometimes you need a certain mutation more.

 

Ohh sorry, I meant can not cant. So it was a joke.

It can. You have a building / unit you cant use anymore (barrack / zergling), and then you sell it / evolve it, so that you get a new unit or can use the money for a new building / unit.

You could still do a lot with them:
- Scout
- Harass (2 - 3 gate and he forgot to block hes scout, 5 lings can then easily take out tons of probes)
- Micro them with the roaches against the zealots, so the roaches tanks and the lings do the damage. Lings are one of the units with the most damage in the game.