I understand the topic is about time-limits given by a buyer, and the manner/context in which negotiations are taking place makes a difference, but I essentially look at this issue the same regardless of whether you're the buyer or the seller.
I rarely mention time-limits during negotiations because I always looked at any offer as having a time-limit, regardless of whether explicitly mentioned or not. Contract law basically says that offers only remain valid for a "reasonable" amount of time before it lapse, and what is "reasonable" depends on the "offerors" conduct (which is determined on a case-by-case basis)... So if you're not sure how long your offer might be interpreted as "open to accept", and would like more control over the situation, then including a time-limit might be in your interest.
Another basic contract law principle is that once you submit a counter-offer, you are also thereby rejecting previous offer(s) made to you. In other words, and generally speaking, if you decide to submit a counter-offer then you are also deciding to lose the right to accept previous offers made to you, and the other party no longer has any obligation to sell at the amount(s) previously offered. The risk of countering, and then losing the right to accept a previous offer, goes both directions and people have the right to change their minds for any reason.
Not only does contract law say this, but it's also how it works with most marketplaces we use (like GD, Sedo, etc). All offers usually have time-limits, but the exact rules between marketplaces can differ. For example, you have 7 days to accept an offer at Sedo, but if you counter then you cannot change your mind and choose to accept a previous offer. However, at GD, any type of offer made remains valid for 7 days, even after you submit counter offers.
Anyway the above is a just basic contract law that generally applies when there are no agreed upon terms that says otherwise.
In terms of strategy, the context can make a big difference too, as well as whether you have your own personal reasons to include time-limits or conditions. Giving a time-limit can certainly help apply pressure, move things along more quickly, and cut through any BS if you are firm on where you stand. A time-limit might also be practical when you're juggling offers between multiple buyers. Giving a time-limit in a situation like [notify]MapleDots[/notify] might be very smart too, in the event Uniregistry has some sort of policy we're unaware of that makes your offer open to accept for the next 6 months, or something ridiculous like that. I doubt that's the case, but at least MD now knows for certain when his offer will expire.
It's rare that I include any comments with conditions attached to an offer because I like having the option to change my mind without looking like a tool. It's not the greatest look when you say "this is my best and final offer, which expires in 7 days", only to come back 2 weeks later willing to compromise. I mean, it's not necessarily fatal but your "firmness" factor angle just lost all credibility.