A RINO (Republican in Name Only) is a popular, pejorative designation. It has two senses, which are typically blurred in popular usage:
i) A liberal Republican
ii) A candidate who runs as a Republican even though he has no partisan allegiance.
Using the same word to denote two different qualities is problematic. Fact is, there's a real distinction between party and ideology. "Conservative" is an ideological identification; "Republican" is a partisan identification. The two never were synonymous.
Now, I think Republicans ought to be conservative ideologues, but that doesn't mean a liberal Republican is a Republican in name only.
Admittedly, these intersect. For instance, the GOP has a party platform. But they don't coincide.
Michael Bloomberg, Charlie Crist, are Donald Trump are examples of RINOs. They have no party allegiance. They are opportunists who join or bolt based on whichever serves their immediate political ambitions.
There's a sense in which Ron Paul and Rand Paul are RINOs. Their allegiance to libertarianism clearly takes precedence to the GOP.
Is that good or bad? Although RINO is a pejorative designation, some people think it's admirable if a politician is not a party loyalist. That's a commendable mark of independence. He doesn't serve the party bosses or establishment. He serves you! (supposedly)
Here we need to draw some distinctions:
i) Partisan allegiance is bad if your party is committed to the wrong cause (or causes).
In that respect it's analogous to ideological allegiance. Fidelity to a particular ideology is only as good or bad as the ideology in question.
ii) That said, there's a reason why so many candidates belong to a major party. When you have the backing of a major party, massive preexisting resources kick in that would not be at your disposal if you ran as an independent. If, moreover, your party is in power, or will regain power at some point, then you can accomplish more by belonging to the party in power, than you can on your own.
And there's something to be said for being loyal to those who help you succeed. To discard them the moment you can get along without them is thankless.
iii) Ideally, this is how it should work. A candidate is forthcoming about what he believes and what he intends to do (or attempt to do) if elected. He solicits political support.
If the party supports his candidacy, then he's not endorsing what the party stands for so much as the party is endorsing what he stands for. Work for me if you agree with me. Donate to my campaign if you agree with me. Vote for me if you agree with me. If my political agenda furthers your interests, support me.