Bigger Party independece/Responsible for constituency instead for the party
An constituency-MP could value constituency interests before party interests
He see hisself as personnaly elected, independant of his party belonging
and have a reduced care for party disciplin.
Composition of Party Members in Parliament:
A Party list is build regarding criterias like
proportion of candiats fo each party wing,
region, religion, gender, minoritiy, ...
This can be counterwised by the in the end elected constituency winners.
Good Chances for Party rebells
(finding their votes not within party sympathisants).
A candidat of a party could be elected in his constituency,
because fighting for non-party interest, he'll get
constituency votes (but not party votes) from
voters outside the party followers.
For Voters of other parties it could make sens to vote for a candidate of an other party,
to influence the composition of this party's MPs without
helping the party by party vote.
The first and only direct elected green German Member of the Bundestag
Hans-Christian Ströbele in the 2002 Federal Election
was asking espiecially for Constituency Votes (without value for the party)
with same critics to his own party
("Voting Ströbele means torturing Fischer [a green minister and vice chancellor]").
Ströbele, running only on a chanceless list position,
won his constituency 084 Berlin-Friedrichshain - Kreuzberg - Prenzlauer Berg-Ost
with 49204 Constituency votes (31,6%), but only 36073 Party Votes (23,1%).
Feeled Splitting of votes/Educate the voters:
IF a voter thinks, he could help a party A by giving it only his constituency vote
but votes with the party vote an other party,
there is no help for party A.
If the voter would be voting party A as first choice, it could be
a lost vote.
Some electoral systems with MMPR specify by leaving of a constituency-MP a by-election. Without new calculation of list seats.
For the party winning the seat in the first place there is the risk in loosing it,
other parties have the chance to win it.
prohibition of leaving his party:
In New Zealand a party can enforce from list-MPs leaving there party
to give the seat back
(example: Electoral Integrity Act, Supreme Court decision 18. No. 2004)
(see also paradox: by-election and AMS/MMPR).
Strategically it could make sens for a party
not to run with a candidat in a constituency
voter, thinking about (supposed) splitting their vote,
are enforeced to do it right.
All MPs would be elected only by party list
and there are still no constituency votes that could interfere with party interests.
The Party has the power of party composition within MPS
(proportion of party wings, regions, gender, gender, minorieties, ...).
If there is a by-election rule,
there is no danger of lossing a by-election,
but a change in winning a by-election.
In New-Zealand there are no constituency-MPs leaving their party with their seat.