He understands that the last restructuring agreement terms is not acceptable. I would assume that he was probably being very vocal in meetings, thus certain parties would think that his presence might make future negotiations more difficult.
Leverage better terms? I don't think you have any idea how much Tsipras has alienated France and Germany's public opinions. Now that supporting Greece has become synonym of political suicide, I believe that Greece has effectively very little leverage.
After Tsipras' call to vote No, I would be surprised to see any negotiation happen in the near future.
If Greece defaults, most likely they will exit the Eurozone and this will be disastrous for Europe (Italy and Spain will be exposed in particular).
Financially, it will send a ripple effect and the Euro will devalue as those debts can't be collected, i.e. written off. Politically, it will be the beginning of the end of the Eurozone.
Again, basic knowledge of the current public opinions of either countries on the subject strongly indicates the contrary.
I do not have the background required to have an informed opinion about the economical consequences of such situation.
Just stating the obvious from a pure political standpoint IMHO.
In Greece's situation, the public opinion (by form of referendum) is used to cement their bargaining position, i.e. the people are ready to accept the repercussions of debt default if they don't get a better deal.
Which is why the negotiation is continuing / will continue, otherwise it will be a terrible lose-lose situation for all.
The problem is that the creditors insist on "squeezing blood from a stone", unwilling to reach an agreement that actually works, and the people of Greece insist that they won't accept that. Now, it's often said and true that in a democracy, the people of one country cannot be allowed to force their will onto the people of 18 other countries. But the facts are on the table; the Greeks have demonstrated that they won't budge. If the creditors won't budge either, it probably means total default and Grexit, something that ostensibly nobody wanted, but that is inevitable given how nobody was willing to make the necessary concessions to avoid it.
In the medium to long term, the Greeks will probably be better off outside a dysfunctional currency union, and the Euro will either get fixed or dissolved after the next few crises.
Greece was sacrificed to save the wider euro, and the German and French public should be grateful.
When a bank makes a loan there's always a risk of losing money and that's priced into the load (or should be).
'moral hazard' should limit the amount of money banks are willing to lose and so willing to lend to people / companies / countries etc.
The banks and european politicians seem to have forgotten about the hazard and instead bailed out those who lent money to Greece and left the public to pick up the cost.