I thought I would wade into this.... yes, you are potentially giving your business away.
There are two separate issues in your question - one is client access to build files, and the other is to IP. The only reason a client would ask for build files is so they can do any future work themselves or nominate a cheaper third party to do this on their behalf - but that is their prerogative. If a client asks us for build files - we charge them a release fee as compensation for taking away part of how we earn a living (repeat print runs and changes/amends).
IP is a different issue but often intertwined. Asking for build files is not the same as asking for release of IP - BUT, many people can not see the difference. Giving a client build files doesn't mean they then own the IP. IP is giving away ANY use of the material in any way.
We had a situation with a company for whom we did some magazine ads (for the Australasian market). They asked us for the 'artwork files'... when we enquired why? It turned out they wanted to run the ads in Germany...! We had produced the ads originally according to an agreed scope of work - X number of magazine ads for X publications for the Australian and New Zealand Market. They are changing that scope - what they are really asking for is the IP. It isn't about artwork at all, but access to the concept and copywriting - our ideas.
And our answer was - sure, for 10,000 euro we'll release the IP so you can run our concept in a market outside the original scope.
They declined our proposal and we no longer work with them. Good riddance to them - what they were trying to do was unethical (remember a business relationship has too go two ways...).
An architect would laugh at this situation. If you pay an architect to design a home you may receive the plans in your hand - but you do not own the IP. You can not then choose to go off and build 300 of those homes. You can build one home on the site as nominated in the building contract and on the plans.
The key to this is having this spelt out in a conditions of engagement contract. Every new client we work with has to sign this or we won't work with them.
In your situation I would negotiate that you will release artwork files to the client that can be used within the intended scope of work and that release of artwork is not a handover of IP. If they reject this - they are being unreasonable and I would walk away.
Just a note on some comments posted about Logos - these are typically done under an 'open license'. Technically the designer still owns the IP, but agrees not to exercise any rights or seek further remuneration. Have a look at the case of the Nike tick - Nike paid the designer... and then came back years later with a share offer. Why? Because they knew they were exposed to recourse.
I actually disagree with the comments that this situation is not insulting. It is naive of a client to ask for IP and not expect to be charged. I've always found people are quick to give away some else's ideas, income and expertise for free... Sadly, it is a trend amongst marketers to pursue this thinking.
In any situation, even if you sign away the IP - you still have the right to be identified as the producer/creator of the work.
This is all totally different in a employer/employee situation - the rights are entirely different.
Hope that helps.