I'll give a run-down of my basic thoughts on the currently available power selections. Head over to the beta forums to learn more about the additional choices that are on the way.
Alpha:
As you stated, it's pretty useful to fill holes in your build.
Cardiac - Good for endurance reduction, in fact it pretty much solves any and all endurance issues. It also has a bonus to resistance powers in it, so if you have any powers that grant resistance you'll see a marginal benefit in those as well.
Spiritual - If you have a lot of regeneration or heal powers then this is the way to go. While it boosts recharge, I find that many builds are able to have effective attack chains without it. So the main benefit is the healing boost. Of course, if you have a power that you'd like to use more frequently and is on a long timer (like a T9 from your secondary or something) then this can be useful as well.
Musculature - If you have no problems with recharge or endurance on your current build, this is the way to go. The higher tiers will show substantial improvements to your damage output. That's assuming you already have an optimal attack chain. It can also solve minor endurance issues by giving Stamina a boost.
Nerve - Don't take this. Ever. Most builds don't need any extra accuracy, and the defense bonus is insignificant.
Interface:
Reactive - This one is pretty much the usual choice. IMHO it's the only current one worth taking. T4 Reactive Radial gives you a 75% chance of dealing moderate fire DoT with each attack AND a 25% chance of applying a small debuff to resistance. That translates to significantly higher damage.
It's worth noting that Interface is gaining a number of new options in i21.5 and many of them achieve a similar effect to Reactive. So if fire damage doesn't work with your concept, one of the new ones might.
Destiny:
Ageless - This is a possible option if you want Musculature Alpha but still have endurance or recharge problems.
Rebirth - Arguably the most powerful option for a Scrapper. It allows you to put a major boost to your Regeneration on top of your existing mitigation. In almost all cases that gives you a very noticeable increase in lifespan. Of course, it would be foolish of me to not mention that this massive boost to Regen starts with a similarly massive PBAoE heal.
Barrier - I could see a reason to take this on certain builds. If you are a Regeneration secondary especially. This power would allow you to add defense and resistance to your already incredible regen levels to achieve better layered mitigation. You might, however, get better mileage out of Ageless since it recharges your key powers more quickly. The weakest (constant at T4) bonus from Barrier, IIRC, is 5% defense and resistance to all.
Clarion - Not much use on a Scrapper. You already have mez protection. A possible exception would be if you have a friend that you duo with who needs mez protection and is a free player.
Judgement and Lore are pretty much all good. Pick whatever best fits your concept. I tend to prefer the damage pets to the buff pets for Lore, as I mainly use them on trials and there are huge buffs to everyone on those as it is.
@Rorn ---- Blue Baron ---- Guardian
Can someone suggest a direction to go with the Incarnate slots on a scrapper. I get that Alpha is generally used to patch up build limitations like recharge or end reduction. But some of the others I'm a little hazy on. the fire proc seems nice, but then so does the defense/resist debuff, I just don't know which would get more mileage. That's where I figure one of you could help.
Thanks in adance!
Life is pleasant. Death is peaceful. It's the transition that's troublesome.
Isaac Asimov