There's no real catch that I know of. When your contract ends, if you don't sign a new contract (and get a new phone) you should also get a $15/month discount on your voice/text portion of your plan each month until you do sign a new contract.
The new mobile share plans aren't perfect for everyone, but if you use more minutes than data, it sounds like it could really benefit you. One warning I will give you is that if you have multiple lines on your account that both call the same number at the same time, AT&T may flag it as "inappropriate 3-way calling" and if you do it excessively, they may disable your account. This can be resolved if it is a legit reason, other than bypassing their 3-way calling process.
The new mobile share plans aren't perfect for everyone, but if you use more minutes than data, it sounds like it could really benefit you. One warning I will give you is that if you have multiple lines on your account that both call the same number at the same time, AT&T may flag it as "inappropriate 3-way calling" and if you do it excessively, they may disable your account. This can be resolved if it is a legit reason, other than bypassing their 3-way calling process.