The topic was about Herb and why he doesn't want to share design creation with fans or staff or whatever.
It's legal issues. Mostly.
There are a number of issues. The largest is, as others have said, require lawyers. When Battletech was first created, the owners licensed artwork from a Japanese company (one of the ones that dealt with Macross). A different company later then licensed the rights to distribute Macross in the U.S as Robotech. Lawsuits, finger pointing, lots of money spent later, several designs in the game are known as 'Unseen' because Cataclyst no longer has the rights to use the images they licensed.
Accepting submissions from fans (like you) that try to offer designs can cause a similar problem. You might sign away rights to the design to have it submitted...but that doesn't mean that someone else they accept a design with who did the same might not turn around a few years from now and cause them headaches. Maybe that person will use their design (which got published) in their own fanworks, or submit it to another company for example.
There's other reasons though, too. There's already enough bickering between fans about 'author fiat'. Look at the Hellstar- 4 ER PPCs... You let someone send you a design through a Private Message, or an e-mail and they start picking them... "How come Faction X gets this awesome design and Faction Y got this crappy design?" While neither design would be crappy per se...just different uses.
That brings up the way designs are made. The fact that you want to make an awesome assault 'mech is good. Very good- the writers are doing their job. The way the designs are made...they're meant to be 'alright'. They're meant to be 'good', but not great or 'awesome'. In other words, the designs people like Herb design are meant to make sense within the fiction (Davion trying out some Light ACs on one of their favorite chassis for example) but not be
optimized.
The idea behind this is that you, as a player, will have plenty of ways to improve it in the game. For example, if you're playing a Davion and get that 'mech in a campaign, you can say "Those machine guns aren't doing anything for me. So I'll take those off...that let's me add more armour" Or drop some lasers for jumpjets...or whatever.
If they designed (or took your awesome design), there wouldn't be much reason for players to modify or tinker with the design. And, as most fans want to submit optimized designs, fan designs aren't that healthy for the game when it comes to mass produced units. Unique 'mechs such as the Bounty Hunter's Marauder II C are meant to be 'awesome' by the way...but players normally wouldn't get it for their own characters. Therefor you get say, a Direwolf, then get to design your own configuration with what the GM has given you- much healthier.
Finally, they're trying to flush out the choices- especially with Clan 'mechs. Omnimechs are hot swappable, so their configuration doesn't matter that much. You'll note that the Nova Cat and Timber Wolf are both 75 tons...yet one has a larger engine than the other. There's little point in designing another 75 ton Clan Omnimech that goes ~86Kph or ~64Kph simply because you already have those 'mechs in the Timber Wolf and Nova Cat.
In fact, if the Timber Wolf and Nova Cat had the same engine and internals, just changing a bit of armour allocation and you could quite literally load the Nova Cat out with the Timber Wolf config.
That makes the different 'mechs pointless. What's the point of new 'mechs? Why take an Atlas over an Annihilator if changing an atlas variant to an annihilator variant was just as easy as changing one atlas variant to another Atlas variant?
So the writers (Well Herb) will look at it and then say "Faction X has no 95 ton 'mech that goes only 32Kph" or "Faction Y doesn't have a 65 ton non-Omnimech that goes 86Kph and has jumpjets" and they work from there.
Finally, they don't have a lot of time on their hands- many of them (as I recall) work a 'real' job in addition to writting. Herb, when he was one step down (Assistant Line Developer) was, in fact, working at Home Depot I believe.
Their time is valuable enough already- from what I understand, the contests to design a 'mech (where it was a competition and most of the details already flushed out- making the legal issues less so than otherwise) was a real headache for them.
That, combined with all the other reasons, is enough for them to do it in house.
I'm sure they have other reasons too....those are just some of the ones I remember reading about from them on here at one point or another.