Although they are not effective in combat when you compare them to the more damage-oriented heroes a healer is useful to have in your team. Healers can be trained and Promoted and are able to heal and bestow quite a bit of health once every other turn. To repair non-organic characters like droids requires a different kind of healer.
If you take into account the linear turn order the game employs you can plan your attacks to defeat enemies before the they have a chance to retaliate, the less attacks on your team the more chance they have of surviving to the end of a mission.
Until your team are prepared don't be tempted by the Bounties and the Cards you can get by entering the arena. It is more beneficial to be patient and train up your team and use energy on Promotional or Training Items.
If you have Promotional Items that only work with certain characters and at certain Tiers and you know you are never going to use them there is no point keeping them. You might as will use them for training or sell them.
Save paying more in the long run by using as many Training Items in a session as you can as subsequent Training sessions get more and more expensive. You can have a maximum of five training items each time.
It is vital that you keep on top of training otherwise you will struggle when you reach the end of the first series of campaign missions and have no chance of surviving the very first mission in the next area (Kashyyyk).
Unless you need the experience, money, or inventory space try to save at least one of each promotion item you find for when you start getting your team to their maximum Tier levels. When you reach the higer Tier levels there is an assortment of multiple items that you can promote so these credits will come in useful if you have them available and it saves you having to look for them.
Do not waste credits on buying Hero Crates when you have built up a decent team, the only exception is when there is specific hero you want otherwise it is more beneficial to save your credits and using them to improve the characters you already have in your team.