Should people play all the Demigods at least a little?

This is the correct answer in just about any competitive game with differing sides/factions/classes/etc.  Understanding the limitations of what's being used against you is the best thing you can do to beat them (or, if nothing else, the best thing you can do to prevent silly claims of imbalance).

Plus, there's only 8 of them and the skill trees aren't that complex.  It doesn't take very long to get at least a working familiarity with all of them.

I focused on Erebus and then got nerfed... yeah all the demis

yes, try all the Demi's. You might find a new favorite. I used to love Erebus, but now I'm really digging sedna and hybrid TB.

Mix it up!

This.

I play Erebus pretty much exclusivly. I'd try other DGs, but I don't really play the game often enough.

Play them all at least through the single player "tournament" if nothing else so you know the moves and such.  Then focus on one till your great with him the start to diversify again.  What I do and seems to be great, I know what everyone can do but I also am really good with one hero.

It's a good idea to atleast try them. You can pick up visual cues for various abilities easier if you used it before. It also gives you a better idea of what each brings to the table and how to compliment them or combat them.

What he said.

I've been slowly completing a single player tournament with each Demigod to learn their basic moves/abilties. Really handy when fighting players online because you know what to expect when fighting them and you can make a better call as to when you can risk a direct engagement and when you should run.

You definitely want to know what each hero can do at each level, it can easily turn a death into a kill or an escape if you can know in advance that TB's fireball is going to do 1050 at 10, instead of just knowing it hits for a lot.  It's even more important to know when it comes to chaining disables and such, so you can activate them at the right time instead of wasting potential stun time or giving your opponent a chance to respond.

Thankfully, you're playing Demigod, and not DotA, so you have a lot less abilities/item combinations to learn .  I currently have memorized the function, mana cost, and typical build order for 96 heroes, so 384 abilites in DotA.  Add in the 60+ items, and then juggling armor equations and whether an ability is physical, magic, pure, a mixture of the two...  something not easy to pick up.

Quick and dirty way to do it is figure out what your favorite one can do to counter what they do, and what doesn't work so well against each particular one.  For example, fighting Regulus as Rook is annoying unless you've got the towers going along with your passive shoulder/head upgrades.  If you were to try to boulder/hammer him, he'd just run away, unless you built yourself itemwise to make up for it. (10.5 speed Rook FTW!)

I'm kinda rambling at this point, but yeah.  Learning what your opponents abilities are, what they can do and how often they can do it is crucial to being able to plan ahead and know whether you should fight or run.

Most of the time you dont need to play them to gain the experience. Once you fight against several builds you have an idea of how they work and how to play one if you ever play them in the future...

I'd say its important to know the threshold of what each demigod is capable of... knowledge is power after all... it will stop you from panicking in a bad situation and running when you could easily turn around a situation...

Try each demigod out a little (at least in single player, if you don't want your stats to get skewed) and keep an eye on the strat forums to see what people are capable of and what builds they are likely to run

I'm not one of those players that panic. After you fight one person in a game he will most likely try the same strategy a couple times and you will no the cooldowns and effects of there primary abilities. I may not be as efficient the first couple times I fight different builds but I also wouldnt have wasted hours in singleplayer...To each his own though. Stats are scewed already from massive disconnects and bugs.

Try them all and try different builds/item combinations.

You can learn a lot about what a potential enemy can do and how to best counter certain strategies.

I've tried everyone at least once except Queen of Thorns, who I've honestly never tried. Her and Rook I've got the least experience with.

It's great to try out all DGs and learn what lvls their major skills and upgrades are by their lvl. That way I know if there is a lvl 5 regulus I am safe to run away at about 700 hp from snipe and I will not get killed because I think it does around 690 at max range. That's just one example.

If there was a random demigod selection then I'd have good experience with all of them

If they'd get around to adding a random choice then I'd be forced to play some of my lesser choices (Rook)

Yes.  Can't wait for random DG selection