Do we really need much more anti armour though? We have: S6 stegs. S5 templeguard, with AP banner they're effectively just as good as stegs, just more attacks. S7 cowboys. S7 regular kroxigors (units of 8). Magic. It's an odd units though and I agree with you that I'm not quite seeing what they should do. I mean, if all we want to do we've already discussed in other threads that regular skinks can do just fine, so what's the need for the krox? If I want a tarpit with a bite I think I'll just spend 250pts on 50 skinks + 100pts for poison. It's still expensive though. The only reason I see us fielding skrox units is if we want to have fast (M6) moving Core units who can flank whatever is hitting our templeguard unit, but in the end I still think I'd rather just pick up 30 saurus with spears. **EDIT** Oh yeah, the krox gives the unit Ld7 instead of the puny 5. This is at least something worth considering because even steadfast Ld5 isn't particularly fasinating.
None of those are core, though. If anything, Skrox units make our core a little more diverse. Also they look cool. If you are playing MSU and want a ranked combat unit, 16 Skinks + 1 Krox might be better value than 12 Saurus Warriors. Skrox also give you a viable option for a core combat block if you wanted to run a No-Saurus list. It probably depends on what you are trying to accomplish with your army. If you're building the most competitive army possible, then there probably isn't room for Skrox in your list.
What I find interesting is the more I play, the more I'm shifting to Skrox. At 300 points, I'm really happy with 24 skinks and 3 krox. The krox hold up a lot better than people give credit for, and with 6 ranks, I'm breaking steadfast on a lot of units. Saurus need help; magic, a hitting hero etc... For the cost of 30 saurus with spears, I'm bringing 8 ranks instead of 5-6. That lets me hold with steadfast long enough for something big and scary to come into the flank and rescue, which is something that saurus don't always do. Also, you only get purpled sunned so many times before it really starts to suck.