(Core Set Perspective)
Like many things in the Core Set, when you first use something you'll think it serves a certain, necessary role. Then, as time goes on those views will most likely change. I used to think Lemuria Codecracker was an indespensible bit of kit that the other factions were at a severe disadvantage for not having; now I just think Gabe's about to give a credit card receipt to a door. I used to think PAD Campaigns were fragile little driplets that always needed protecting with at least three layers of ice. I used to think Desperado was bollocks.
In the case of Parasite, I used to think it was a vital part of a faction whose limited ability to deal with ice meant they needed a more permanent way of equalising the field. Now I realise it's just one of the many ways for Anarchs to be bunch of caustic jerks. Which is good! Because in my opinion that's one of the things they should be - a chaotic force of disruption that grabs the corp's carefully laid plan off the table, eats some of it, spits out some more, rolls the rest up into a makeshift kazoo and then parps loudly in their sad little ruined faces.
Parasite is your ticket to achieving that in the Core Set, one of the key things that gives you that malicious edge. Especially when working in tandem with good old Datasucker, you are able to not merely overcome, but actively wreck the other players defenses with worrying speed. Couple that with Noise's ID ability (frequently triggered by Parasite), and the ability to recur (get back) the little bastards with Déjà Vu, and you have a system of attack that not only threatens what they are hoping to get into their hand, but also weakening their ability to take advantage of it once it does.
Functionally, this all translates to agendas getting bunched up in HQ, being thrown in the bin or going down without adequate protection. With the Anarch breaker suite being as good as it is, you could Shaper your way to victory quite happily, but where's the fun in that?
To finish, here's a few things I've noticed during my time with it:
Rezzing big ice early is usually a solid investment for a Corporation but against Parasites it becomes a potential waste. Do you rez it early and risk not finding the cards you were hoping to put behind it, or do you wait until they show up? By which point you might not have the option.
It might be tempting to hold onto your Parasites for juicy targets, but the earlier you use them the more times you'll benefit from Noise's ability to trash cards, and you can always get them back later. A middling piece of ice in the right place at the right time is much more valuable than a big piece when it doesn't matter.
Having Grimoire ready for Parasites (or any viruses) is ideal, but don't do what I did and have it hold back your plans if it doesn't show up. Waiting for the ideal set-up can and will cost you games. The nice thing about Parasite is it will always be useful in some way, even if it's not quite as potent as it could have been.
Wyrm is aways going to be a little unwieldy, shall we say, but with Parasite you can at least give it some meaning. While it's not as strong as Datasucker for bringing ice down to be gnawed to bits, Wyrm doesn't rely on virus tokens and will keep the danger of your little green nibblers present even after an annoying purge.