Assume that in order for the shard bombs to be "worth it", they would have to out perform other alternative weapons which for the most part means to outdamage them, given that the bulk of the shards deal nothing but damage. The biggest competitors, and the ones that they should be balanced with are nitronome and dark briar barrage. These offer the most "standard bomb" type approach.

So lets compare and contrast these weapons. All numbers are according to the official wiki, and are assumed to be at D28.

-Dark Briar Barrage-
Actual: 394 (Actual = Neutral + Monster Defense)
Effective: 312
Neutral: 230
Damage Max + Effective: 406
Damage Max + Neutral: 342

-Pure Damage Shards- Damage: Core/Shard
Actual: 239/263
Effective: 143/186
Core + Shard; Effective = 329
2x Shard; Effective = 372
Core+2x Shard; Effective = 515
Damage Max + Effective: 214/244
Core+2x Shard; Damage Max; Effective: 702

Neutral: 75/99
Core + Shard; Neutral = 174
2x Shard; Neutral = 198
Core+2x Shard; Neutral = 263
Damage Max + Neutral: 132/162

The damage dealt by the first shard bomb dropped is:

C * numC + S * numS = Total

Where
C = Core Damage
S = Shard Damage
numC = Number of hits by the core
numS = Number of hits total by all of the shards combined.

So lets play with this a bit
If 3 mobs get hit by the core and 7 hits are recorded on the outer shard, the total damage (at max effectiveness) will be: 2350

...Which is the same as hitting 5.8 enemies with dark briar barrage. This damage isn't bad, but it isn't outstanding either. I can hit 6 enemies all day with dark briar barrage. But the bomb would still be better if you perfectly hit 4 or 5 enemies with it.... So where does it really ramp up?

Lets say we REALLY shard them good. 5x Core + 15x Shards. That'd be if we vortexed them into the middle, and perfecly shot them out and then hit 5 other enemies with it somehow. Like a huge crowd of slags. The total slags to make this happen? Around 10 or so. We would do 5015 damage with this hit---- the same as hitting with 12.382 Dark briar barrages! :O If we pulled this off with only 8 slags, Dark Briar Barrage would have only dealt 3240 damage to them, making shard bombs very much the better bomb in this scenario.

However...
This only applies to the first bomb or so, because damage after that doesn't always count

Zeddy, who I must mention at least once in this post, discusses how the invincitinks work, and why we have them, here:

http://forums.spiralknights.com/en/node/82883

If you notice in his video, the bomb gets cut off after one use. This means that on average while getting in many hits, the bomb deals far far less damage than dark briar barrage. As shown above, shards would easily become the damage bomb of choice for experienced bombers, but in practice there are a number of issues with just slapping enemies for 2x shards + core repeatedly.

First, the bomb isn't exactly party friendly at all. If you've ever been in a party with someone spamming the thing everywhere, you'll know what I'm talking about. Random knockback, combined with huge numbers of flashing lights and a dash of lag make the experience truly awful. To use the item effectively, and not find yourself solo wondering what happened, it takes a great amount of skill.

Second, the calculations I did were the absolute best case scenario for the shard bombs: Max damage against all effective enemies. Against neutral targets, or even mixed targets, the damage of the shard bombs really falls. 5x Core + 15x Shards vs Neutral targets at no damage boost = 1,860 = 8 hits from DBB. Using the shards at that point is largely pointless.

Third, chances are we aren't going to get in such nice hits all of the time. Against 8 slags, we're more likely to get in a bunch of random combinations of these hits. Dealing 405 damage per enemy basically means 2 hits per enemy. If we do less than that, we're losing potential damage that Dark Briar Barrage could have dealt.

While we're on the subject of crazy damage...

Acheron Charge; No damage increases vs effective: 1,712 against a single target, if all hits connect.
Acheron if all splosions hit enemies twice and the initial charge hits 3 enemies: 3,900

Dark Retribution; No damage increases vs effective: 83 * number of hits;
If each orb hits 3 enemies 5 times = 1,245.
If each hits 8 enemies 7 times = 4,648

I would say that, given the assortment of data provided, invincitinks on shard bombs are almost entirely unnecessary. Plus, as Zeddy wrote in his post that I linked above, shard bombs being terrible is even likely the result of a bug.