Yeap the battery needs a charge if the bike is to start up. DO NOT keep trying to start the bike with a dead battery!! You can damage the rectifier they say. And it's just a waste of time. The battery will suck any power generated by kicking it - keeping the plugs from firing. I LOVE those Battery Tenders and I wish I had one of those when I was a kid! All I had was a medium sized charger and you can't even leave a battery on one of those without damaging it. If you put it on a big charger it will melt down - I did that once.
Those Battery Tenders are so cheap now and they work perfectly and prolong the batterys life as well. They make the regular size chargers and they make a smaller charger that is like a laptop power supply which charges slower but if you use it for a trickle charger/ conditioner you don't need anything bigger than that. If you buy the bigger one it works better for charging dead batteries faster but that's all.
I have a 6v and 12v Battery Tender and the 12V works good for car batteries as well. The only pain in the butt is storing these things and all their cables. I just find a box or a small canvas bag to throw them into. Just make sure and keep track of the accessories as they are easily lost. Keep them in the box or bag you use for storage or you'll never see them again.