Worth sniping on Legendary Banner?

If you don't care which 5-Star you want--or you have more than one unit that you do want--from the Green and/or Red pool(s), then it is absolutely worth sniping for those colors.

However, if you are aiming for only Elise in the Colorless pool, then you should not snipe Colorless and wait for a different banner.

So you want 6 out of the 12 focus units, which means the appearance rate of 5* units you want is 4%. Compare this to:
- a 3% banner with three focus units on which you want two of the focus units. Assuming you don't mind getting half the non-focus 5*s instead of the unit you were going for (pulling a B.Ike, for instance), the appearance rate of 5* units you want is 3.5%.
- another 3% banner with three focus units on which you only want one of the focus units, and same situation with the non-focus 5*s. The appearance rate of 5* units you want is 2.5%.
- a 3% banner with four focus units on which you want two of the focus units, and same situation with the non-focus 5*s. The appearance rate of 5* units you want is 3%.

Looks like you're getting a better rate of desired 5*s than just about anything else. However, you still want to consider how many orbs you want to leave behind toward saving up for other banners (CYL, for instance, a popular target).

Generally, anytime you want all three or even 2/3 on a colour, you're getting a good offer as far as the number of orbs mathematical probability expects you to fork over for a 5* that you want. However, the offer will not always good enough, depending on whether or not you're ok with not getting one of those two units and what else in the following weeks/months you want to target using your orbs). You might want to have a look at a current question about firing now or waiting for the Hector+Matthew banner.