Investing in a MotoGP team, newly designed bike or rebadged, will still cost major Euros. And Piaggio wants to (eventually) see it's money back...
The examples you use, Laverda and Buell, weren't (or won't be) very successful.
- Using a new MG race bike will have to result in a new MG sports road bike, and that extra risk (very high investment) Piaggio will not take.
- Rebadging just won't do. Then they'll also have to sell rebadged Aprilia's (RSV Mille or the likes) with the risk of alienating their current target group in which they already have invested a lot with all the new "touring" models. Plus they risk losing the younger Aprilia riders in their customer base.
Maybe not in Germany or Holland, but how about other countries?
But I agree with you: MG competing in MotoGP or WSB again would be great!