I've had a single egged priest on foot kill 10 HE archers alone.
I don't think the guy is useless, not at all, and there are plenty of ways to suit him up nicely.
:
The ASF sword.
The Egg.
Enchanted shield makes a cheap but durable chief.
Cheap magic weapon (your pick) for a magic attack.
Basically he goes well with a lot of stuff that we normally wouldn't use on our Scar vets or Old bloods because it would take away something from their ton of attacks and durability - the little guy is much cheaper, and hence much more suited for a glass cannon kind of role. Even dis mounted he can be outfitted with a 3+ save (some armies have to live with that, it's weird) and retain great movement with MS 6.
For mounts :
Both flying mount options, even the Stegadon option is tempting as a BSB, being more durable than the Carno, and at about the same price range.
LD 6 isn't impressive but it's better than a normal skink, and it's in the "will mostly make" category, sadly not in the "should mostly make" category.
I don't think there's a big enough downside to him, since he went down to 40 pts, that i wouldn't field 1 of these guys at 2.5K, heck i'd try to squeeze him in at 1K just for a love of the models, but he'd still be useful.
The thing is, he clears chaff, he clears warmachines, and he does that well. But he WILL die (on foot at least) to any decent unit, if he's on his own (probably to shooting as well unless you can keep him in combat).
In a unit he loses the Chaff clearing, warmachine clearing role, but gives a Cohort some needed punch, and he ain't bad at it. the 3+ save isn't too bad to make your opponent focus the unit instead of him either.
On a mount he still clears chaff, hunts warmachines, and can relatively safely flank units of core models, even some special.
He isn't the "kick all stuff i meet" kinda guy we know from a cowboy, or "anchor my saurus pl0x" kinda guy we get in a footed saurus with CoC or BSB, but more like a "glass cannon toolkit, pick your own pieces and needs and go".
To summarize :
He's our expendable glass cannon kit.
Whereas our normal heroes are less expendable but more tough. They hit harder, they cost more, they do more, they're less mobile.