The runic pet is a caster dps pet and doesn't generate enough aggro to hold a mobs attention. It has the roughly the same health and can actually handle a physical hit better, but its not designed to snap aggro off you the way a melee pet can. If you are outside a raid or not killing a high health named, you will want to keep the relic pet out.
I like Dread Lich, the only issue I have with it is I think that the return should be 8-10% higher resulting in 33 - 35 % return. If you aren't in combat then you don't need it. You have lich to regen mana and even in a group a healer should be happy to pop you a hot to keep your dps stable at a higher level. I use it on raids when taking 5 k aoes and that is enough to keep me up. Your lifetap should be hitting for @ 1500 base and that should be fine for keeping you alive if the healer is bad. Though really if a healer won't toss you hot, ask them to send me a tell.
To understand why insidious infection got changed, you would have to understand how HB worked initially and how it worked at the end. Initially, HB at tome 4 had a unlimited chance to spread with a 40% chance to reproc for each ticking element for each mob in its range. I actually got to think of it as an aura (lols), which it was when it was up.
Combine that with a sk casting veil and you xp $$, because the sk could pull 20 mobs in citadel and I would blow them up. I won't get into kiting remnants where it was possible to basically pull and kill all the non-summoning mobs in parts of the zone at one time. I did this a couple times, glorax did it a lot. It resulted in DF a third of the time because of stuns, but the other 2/3ds was fairly op. Now the real money was to box fuwok and pull 40 -50 mobs to him, you didn't need a healer, he never dropped below 80 % and once it got rolling he was 100 % until the end.
So for this and other things HB was made into a a 6 target aoe with no chance of reproc. At this point, II became meaningless with not working at all well with scitterpox. So it was change to what it is now, which is really good for necs and is part of our heritage of having significant spell penetration.
The mana group return on sap was changed because it was possible to aggro greens and insta-fill your groups mana bars. Really this probably blows at lower level play but its not a huge change later on when you will be killing more high health mobs. It was probably also changed because it was possible to complete heal and mana groups with HB under what might be otherwise trying circumstances. Also, think of the result scaling to difficulty which is more appropriate. Also, without the old HB, this is kind of a once in a blue moon thing since we really aren't set up for kill shots anyway.
The pet mimic tap hasn't changed. Which is fine, its really a cool pet toy rather then any dps. I just use to see if i can time toby getting the kill shot in groups for a little mini game. Its pretty uncommon, but kind of neat when it happens. Its of little use keeping the pet alive since twelve seconds is too long to wait if he is in peril, if it even procs. it does work well if you are kiting mobs with a damage shield to keep his health up, though i really use a lifeflow for that. Also,charm pets proc the tap, which is funny but results in brief but significant degradation of dps.
Also, toby has wiped a raid with it which was hilarious since he was on pet hold. Yeah, the proc works through pet hold lolol. Every one cheered up when bango asked who the dipshit was and I said Toby.
** Zaela is correct as well.