The need for an exit plan seems to be mixed from investors.
Since most VC's tend to discount the accuracy of financial projections in a business plan, they to will discount your ability to accurately forecast a planned exit. Rather, investors will focus more on the likelihood and probability of your firm being able to reach a certain size within a certain time period. If your firm has a high degree of probability for high growth and profitability, many investors assume that once growth milestones are achieved that an exit with an attractive rate of return is inevitable.
Just look at still private firms like Facebook who have kept their investor money private for much longer than many anticipated. However, a public exit via an IPO is inevitable for Facebook and investors will ultimately be rewarded with a handsome exit. When Facebook initially secured venture capital, they weren't pitching an exit, they were pitching the ability of Facebook to become the most popular website in the world (which they just surpassed Google). With that sort of promise, a handsome exit for investors is inevitable.
If you are pitching to investors, it may be worthwhile to talk about your "proposed exit plan" with the contingency that this is very much up the in air as with everything else with your business. However, it indicates to investors that you and your founding team are thinking about an exit. The anticipated exit requires very different strategies from the early stages of a business. If a firm requires an IPO as an exit, as many early-stage biotechnology and medical device companies than their strategy is solely focused on building a sound R&D strategy and entering into clinical trials as soon as possible.
Firms that are not as dependent on early R&D investments to fund later profitability will have other early-stage strategies more likely focused on revenue growth. The faster you grow your top line, the faster your valuation increases (just look at the .com bubble in the late 90's). Many entrepreneurs seeking funding think that they have to pitch their potential investors on their exit plan. Most new ventures state that they will be acquired by a competing or complimentary firm within 5-7 years of starting. That "exit" event usually never happens as planned and can therefore almost never accurately be planned for, but is should be prepared for.
Rather than focusing on trying to predict a future outcomes such as acquisition, focus on selling the numbers and growth that your business can achieve to potential investors. State your desired exit plan, such as we don't want to do an IPO, we would rather be acquired by a strategic partner and thus we will be seeking formal partnerships with potential acquirers early on in our venture to build those relationships. Leave the quantitative numbers out of the presentation, but show that you are committed and have prepared for an exit at some point in the not too distant future.