As always it depends. This is not legal advice. I am not a lawyer. However I also think that if you have to go to court, you have already lost more than you will get back. The most important purpose of a contract, IMO, is that it provides a joint reference for expectations. Keep in mind that typically a work agreement (contract or set of contracts) is going to consist of essentially two parts:
- Agreement of terms of doing business
- Agreement of scope of work
I personally like to keep these separate. This means a basic work contract does not include client needs, but the whole bid/approval process does. This has a few benefits:
It keeps lawyers on both sides focused purely on terms of doing business, not on reviewing scope of work, and
This helps ensure that additional changes to scope are easier to approve.
Now typically a contract is considered to have a number of essential elements and may be explicit or implied. These may vary by jurisdiction but typically involve an offer for an exchange, an acceptance of that offer, and a general agreement on what was offered.
So my preferred approach is to include in the contract a section which just notes that scope of work will be provided by the customer, and bids will be dependent on that scope, and that changes to scope may involve additional fees.
Then the bidding process can be incorporated by reference.
Remember also that it is not uncommon for a given piece of work to be governed by a series of contracts, including non-disclosure agreements, work contracts, and the bidding/order process. There isn't anything necessarily wrong with keeping these separate, particularly if the people who need to review changes to different pieces are different people. For example the lawyers don't want to be called in because the customer wants a little more done.
So as a summary you want to think about how a given document fits into the rest of the process. There may be many more contracts that cover a given piece of work and these may be explicit (an NDA) or implied (offer/acceptance of a bid). Hope this helps.