True. I know Kamui is a great technique. In fact I was telling my friend just the other day that I would choose to have Kamui if I could have one MS technique in real life. However, this does not mean that he has to depend solely on intangibility. You're saying that if he uses other techniques that he would be open for attack. That can be said about anyone using a non defensive jutsu. By this logic, Naruto shouldn't use rasengan nor Sasuke chidori because they are not defensive jutsus. They are attacks rather than defense. As a ninja defense is important, but so is offense. Obito's kamui can be used offensively, but it is actually arguably riskier to use this for offense rather than other jutsus. This is because with jutsus like wood style, fire style, shinra tensei, asura path etc..., you can attack from a distance and still have rather good defense. With Obito's Kamui, you have to get close enough to your opponent to touch them and you must solidify to actually attack and absorb them (thus giving your opponent more of a chance to hit you or explode paper bombs near you than they would have if you were attacking with rapidly growing trees from a distance, firing missiles or shooting gravitational waves). As for the point that Obito got headbutted by Naruto when he used wood style, you have to remember that Obito only used a very small and narrow branch to try to hold Naruto off. It was nothing in comparison to the offensive and defensive capabilities of stuff like deep forest emergence. Obito was practically begging to be attacked. By the way, the wood style that Bee was in came from Madara. Quite frankly, Kakashi's long range kamui is better for offense than Obito's is. This is not to say that Obito shouldn't use Kamui, but before the war he didn't even eject shuriken. It was just the same old thing every time. Yes it was very effective, but it also almost got him killed a couple of times. All I am saying is, Obito could still phase while throwing in a couple of mokutons or a couple of asura path lasers every now and then. It wouldn't hurt and it would actually help.
PS. Even if he can't use kamui while using rinnegan, shinra tensei only lasts for about a second. There's a 5 second interval before he can use shinra tensei again, but he can still use other jutsus like kamui immediately afterwards. He can probably actually use both rinnegan and Kamui at the same time because he used chakara chains and kamui at the same time and chakara chains are an outer path ability.