In last 10 years of SEO, I have had only one client site give a link to my site and that client was my classmate. I haven't asked clients for links, since they are paying me for my efforts and are under no obligation. But, if one chose to do it, is it a wrong strategy, ethically and/or technically? I beg to differ -
1. Ethically - In almost all print ads you will see the ad agency mentioned. How different would the mention of the SEO company on its clients' sites be?
2. Technically - I would equate link to an SEO company as a link to the web design company. I am yet to see a site that is adversely affected because it has a link to its designer. Or an e-commerce site because it gives link to its shopping cart software provider.
However, I agree, mentioning one's SEO is not in the client's favor and there should be a no-compete agreement, should the client mention the SEO.