I play a mage and have obtained 100 focus via crafted items and a blue ring. Should I bother with t1s other than to experience the content or move right to t2s?
The hit/focus caps are 50 for T1, 100 for T2 and 200 for T3 (see: http://telarapedia.com/wiki/Focus). So, basically your focus rating is enough for T2. But there are several reasons to start with T1.
#1 Learn to compete in expert dungeons!
You'll will use an other soul combination for expert dungeon runs (*1) or you'll have to fulfill a new role as healer (chloromancer) or supporter (archon). T1 runs will give you the opportunity to gain experience in this new soul combinations/roles in the more forgiving environment than T2 expert dungeons.
*1 for example Pyro/Chloro/Elementalist for questing and Pyro/Warlock/Elementalist for expert dungeons.
#2 You focus may be enough but whats about your spellpower and/or crit. rating?
Not only focus is relevant to compete in T2 expert dungeons. T1 equip (you can buy for the expert marks) and T1 drops will help you to raise other important stats.
#3 Recipes drops
There are some T1 recipe drops. If you're a dedicated crafter T1 runs are a chance for you to get these.
If you're up to it.
My recommendation is that you should do some T1 expert runs to improve you character and gain experience in your new souls/roles.
I'm unable to find a source at the moment, however, Trion has stated that Tier 2 Dungeons are intended to be completed with equipment from the Tier 1 Dungeons.
So yes, there is a reason to do the Tier 1 dungeons: so you can be properly prepared (and geared) for the Tier 2 Dungeons. That said... if you can do the Tier 2 dungeons fine as you are, I don't see any reason you can't just do them now.
There was a T2 run I did with a healer who never ran any dungeons, and he crafted all his gear (Focus 100+)- he thought just because he met the stats required for a T2 run he could handle it.
He could barely keep up with healing. As you know, chloro heals rely on damage output, so spell crit and spellpower play a huge role in the return on heals.
We eventually got through it, but, you need to be raid/dungeon aware. No one likes lengthy T* runs or wipes.