i have levelled 3 cabs on live each fullspecced in one of the 3 lines - i also got a cab here (spirit-spec, though not 50int it) and here's my summary:

matter:
great for mass-pulls with focus-shield and then ae-dot (sapph-pet is the way to go)...specs and tactics have been told here quite a lot but i wanna add an important thing: there's only very few spots, where you can actually DO such mass-pulls fast enough...pygmy-goblins are like the only useful mob to do this - maybe dunters as well but iirc they are matter-resistant, which makes it harder here ...usually after 1 pull the spot is cleared and takes several minutes to recover...solo you can only do this with blue mobs, so maybe it's better to switch for high yellow or low orange mobs, which should be doable with focus-shield (haven't tested that on uth, but worked on live)...maybe you can switch between a yellow and a blue spot...pulling yellows singlely while the blue spot is recovering and farm the blue spot inbetween

body:
bad spec for levelling...it's really not made for pve, so better not use it...the lifetap is too weak to really be effective, you don't have enough spirit-spec to usefully recycle your pet (takes like 3-4 cycles i guess) and without dots you'll have a hard time once the mob is in melee (with dots the mob might die even when he is able to hit you 2-3 times before)

spirit:
when i levelled my spirit-cab (went spirit from the start) pet-buffs were broken and only dmg-add + haste worked, so i can't really tell how a buffed pet does in pve, but after the fix of pet-buffs it does like 2.5x the dmg in rvr and takes only about 50% of the dmg (if not less), so yes that should make a difference...until you have the yellow debuff just use sapphire-pet and let it attack a mob...when the mob is at 60% or something you might as well start nuking...that's how I did it, but then again i only had to rebuff 2 buffs after recycling my pet...maybe as spirit-cab it's best to just let pet hit the mob and apply the base dot or nuke 1-2 times


special tactic with ruby-pet, spec-independent
if you level in a wide area you might wanna try something I found very nice due to bad pet-melee dmg - the ruby-pet-nuke was not broken when i levelled so basically the nuke did the same dmg as for example 10-12 melee hits, which IS a major difference...
important thing is to let the pet "stay" on a spot at least 700 locs away from you, better 1000 - basically there's 2 things you can do: let the pet pull with nukes and when mob is in melee just nuke it down yourself...not pet-buffs except for dmg-add needed so you should do fine with recycling (maybe heal pet once before cycling, otherwise you might have too few mana to cast new one afterwards)
other way: you pull a mob yourself with 1-2 nukes or dots (depending on your spec, make sure at least ONE spell was NOT resisted) and then set pet to attack the mob...

the first way is a little slower, as the pet might take a while to take down the mob in melee if too many spells were resisted...the second way is faster but also dangerous...if your pet has too many resists the mob might kill you, before it gets aggro

this technique should be used on yellows (maybe not high ones) or high blues...never do it on oranges, your pet will have massive resists there!...after a while you'll get a good feeling on how to do this...i guess with matter-caby it's even easier...let ruby-pet pull with nukes, and when first nuke did dmg, apply both dots...together with the rest of the pulling nukes that might work fine...


that's just my experience from uth-levelling with a broken spirit cab...on live i just focuspulled most of the time, though downtime was pretty bad...best was levelling in grps and applying ae dots + single-base-dots on all the mobs, once the tanks had enough agro