In general, I side with the 'No' vote...
This is because I my last Preset build session, I had literally thousands of IR options available to try in addition to the stock ones. GM Arts is of course quite right in saying that you need to try one then give your ears a break because you're always hearing the next one in comparison to the last - that's a tough conundrum and if I took a five minute break between each IR change, it would take days, even weeks with no sleep to work through them... and you can bet that by the time that's done, I'd have no longer have any idea of what I was looking for in the first place
Of course, some of the options could be discounted as 'inappropriate' from the start. Looking a 4x12 tone..? Don't go looking at 6" ovals etc but all the same, that might actually produce something golden!
As Clark says above, an IR is a sound shaping tool. That's great, but most of what I waded through kept shaping the tone in the
wrong way
for me. Now, that's not to say that there wouldn't have been options in there that suited song A or song B a little better within the context of the full band mix, but finding that wood amongst all the trees is a hugely tiresome and often fruitless process.
I have two or three IRs that generally work for me and suit what I personally look for. Despite entering the last session with a completely open mind and willingness to find more gold, I ended up back where I started because they just work and the advances in the Amp Block give me all the tonal flavours I need.
Having now rambled on about that, I actually love that the options are available for everybody - everyone is different and what tickles one person can turn the next one off etc... but it is indeed a deep rabbit hole and with the stock options covering a lot of ground (yes, two of my three final choices are stock) I can help but answer the original question of "Are Cab Packs necessary" with a 'No' - but caveat that with a 'Who knows, your personal heaven made lie deep within some obscure pack somewhere!'