Two things.
1) Yes, well designed multi-parts are much better than poorly designed ones. Ball sockets, cylinders, good contact patches help enormously.
2) Multi-part minis represent a higher barrier to new mini fans, regardless of how well designed they are. It's frustrating to the newer folk, or those who don't get as much enjoyment out of assembling minis, if the only way they can get a favourite or factionally-appropriate mini is in too many pieces for their comfort level.
3) And there are too many poorly designed multi-part minis. Mainly historical, but I'll point to the Lament. Separate feet & legs, but there's no point, because they're not posable - the foot contacts the ankle with a flat plate. And the arm attachment points are tiny, and don't allow for posing. And that's a modern one, not the Reseen Thud or original Reseen Marauder.
I really appreciate you're thinking about these things. Two questions in return:
a) What is your main reason for going many pieces over less?
b) What are some of the sculpts you've produced?
To answer A) I'm also going to address the "barrier of entry" statement that was made. The Liya produced single-piece miniatures that CGL produces is extremely accessible to BattleTech players. I will grant that not every mini that IWM makes is made by Liya, but if barrier of entry is a concern, then the plastics is where those players should go and miss out on a few miniatures.
The mindset that I've been encouraged to have by Anthony (and who I agree with on this) about parting the plastic miniatures is that the IWM offerings are for the more serious/committed hobbyist that will want to have options. That they are a higher-price point offering and should come with the ability for greater control over the the pose of the miniature to reflect that, along with the cleaner casting provided by pewter over PVC. That's part of it.
The other main driving force of it is that the alternate loadouts, requested by IWM (variants/omni configurations) require additional parting to maximize reusability of existing parts.
Finally, the most important factor is castability. For instance, it was specifically requested by IWM that the Rifleman's side torsos be parted off because of casting concerns.
I've done the sculpt/parting work on the
Cyrano VTOL
Amarok
Viking IIC
Inferno
I've done the parting on these models that have gone from plastic to pewter
Night Gyr
Savage Wolf
Mad Dog
Archer
Phoenix Hawk
Nova Cat
Rifleman
Timber Wolf
And I think about half a dozen more models that Speck hasn't brought up yet, to my knowledge so I won't. Some of those models that I did the sculpt work for were in my early sculpting career and I wish I had another shot at for various reasons (Amarok and Viking IIC specifically) but still were generally fine outside of the cracking issue that we had for a few years there.
Finally, I wholeheartedly agree that multipart kits have a history of being poorly done. I don't understand why IWM refuses to cycle out anything, despite how poorly it was made, or how old it was, but also it's not my business. However, the Lament is 10 years old (at least according to the copywrite on the image on the IWM website) so I would absolutely lump it in with effectively anything made prior to the BattleTech renaissance that started with the Game of Armored Combat Box and call it 'old'.