Yes they do have to do compromises, they do them all the time. Compromises on who to cater to and who to piss off...
I am not saying they do not do any compromises. I am saying they do not have to do EXTRA compromises to form coalition. And those compromises could be particularly great.
Btw you can have a party holding +50% of the seats in multi-party democracy too.
That, by itself, is not a good thing, since that would essentially mean that there is no strong opposition. I am of opinion that strong opposition is always needed. It keeps party in power "more honest". This is again advantage of the two party system, as I mentioned before.