Theoretically yes but the shards blow up too slow for that.

Oh, you tested it in the training hall and are theorycrafting its uses? That's cute.

Nitronome, BAB, Irontech all already perform that role and do it at a much higher damage per second, have drastically faster set-up times, have a larger AoE, don't require enemies to get into your personal space to set-up, are more predictable and much more versatile (it works from a corner, around a corner, and over gaps).

DPS & Set-up
Let's put it this way: I can drop a SSS/DCB/SSB then charge and drop a Nitronome and the nitronome fuse goes off before the second shard fuse. Yeah you can push out with the small explosion into the secondary explosion but by that time I would've already killed something with a blast bomb. What happens when you don't do a combo? Oh.. you're just doing 24% less damage than a Nitronome (assuming a shard even hits which isn't guaranteed). The set-up essentially means that in the time I can hit for 638 damage w/ nitro @ D27 you're playing for a maximum potential of 669 (assuming small radius explosion then double explosion) but most of the time it'll be more like 429 damage. Not good.

You have to consider the maximum potential of the shard-style bomb is really only for a small amount of enemies as well. The radius of the first explosion is so small that you're likely to only be able to deal max damage to 1-3 enemies at most. Thinking about the shard-style bomb in terms of high-damage is flawed in this sense. It's never going to do 669 damage per bomb consistently. Honestly, I'd be surprised if you could even do 429-480 consistently.

Small AoE
We're working with 2 radii here and they're both smaller than the Nitronome. Use it as a shield? That requires enemies to get right into hitting range of you for the first bomb radius. The small radii on the second ring makes getting double hits more of a dice roll than a guarantee. (I've actually managed to push enemies into the second ring and have them escape with NO damage!)

Predictability
If I drop a blast bomb I know EXACTLY what direction the enemy is going to get knocked back. The problem with a two-stage knockback bomb is that you know what direction the enemy is going to move the first time but the second explosion could knock them further back or towards you depending on where they move after. This only takes into account a single enemy as well. When you have multiple enemies you'll have situations where the outer ring will be pushing enemies into you. I don't think using the bomb "as a shield" is ideal here. You have no idea where enemies are going to get knocked back and on top of that you're working based on where you think the enemy is going to be 6 seconds from when you drop the bomb. That kind of latency just doesn't work to protect yourself. It makes it impossible to have any precision on positioning. If you make a mistake you're doing no damage, little damage, or getting yourself hit.

The bottom line is blast bombs don't have this kind of latency between bomb drop and payload. I'd say that I can push an enemy in the direction I want 95% of the time with a blast bomb. It's a coin toss with the shard bombs.

Versatility
On that note, even if you were able to push enemies with the shard bomb, once you get it against the wall the outer ring really starts to play against you. If you get too close your shards disappear and you're either dealing 24-40% less damage or NO damage. In comparison once you pin an enemy against the wall w/ a blast bomb the battle is essentially over because now you're consistently doing full damage with EVERY bomb and vastly out-DPSing a shard bomb.

In open ground with or without obstacles the blast bomb is better. Once you get into corners, against walls, around gaps the blast bomb becomes so vastly superior that you'd be a fool to use a shard-style bomb.

Sorry but if I'm going to use an equipment slot for a bomb I'd like it to work in more than just the training hall ;)