Have you tried crouching? I haven't turned the game on in ages, but if I remember correctly if you crouch while dashing you can seamlessly transition into any move or nearly any move. Pretty sure it works with DownB. Hang on, I might as well check. The DS is right here... Well, I guess it only makes a difference with A moves, but still, Klaw looks like it's only slightly quicker and it does less damage while being just as punishable. At least with DownB you might land the second hit.
I dunno about it being "Just as punishable" as down B, I suppose certain characters can still get smashes on you, but that's generally only if they spot dodge the klaw and I think Bowser likes when the opponent is spotdodging. However, Down B has similar issues, and it's even punishable on hit if they are at low enough percents. Also, you cannot crouch during the dash, but Bowser's run stop animation is very fast so you can essentially do any attack pretty seamlessly from a dash. However, I'm pretty sure that all specials, aside from neutral special just because of the fact you're holding forward, can all cancel the run animation directly without a run stop. I don't know about you guys, but I intend to B-Stick once the Wii-u version is out.
In situations where down B wouldn't kill them anyway, Klaw is a completely valid option just for the fact that you only need it to connect once for full damage, and it can be used for positioning. Getting the opponent to the edge or offstage is generally better than doing more damage, especially if it's only a 6% difference, you can easily make that up with pretty much anything else. the less you're using the bomb, the more likely your opponent won't be thinking about it when it matters. It also helps when you're not using bomb all the time because your opponent is less likely to be thinking about it that one time when you actually need it for the kill or the shield break.