On tactical voting, I'd only due it if there was a chance of getting rid of someone I'd didn't want. I wouldn't do it as a protest against some aspect of the party I'd naturally support that I didn't agree with. So, in the case of Labour, I'd want them to be more radical than they're promising but I'd still vote for them here.
They've got better places to spend their campaigning funds on, e.g. Bristol Central for the Greens. As to why bother offering candidates, for a country-wide party you pretty much have to and the Lib Dems certainly see themselves as that, the Greens are probably getting that way, too. Labour will put up people in seats where the Tories get over 50% of the votes, and vice versa, because to not do so would be seen as defeatist.
Anyone, off to work, so that's my tuppence worth on this topic.