HBS professor Mark Bradshaw reports

"The prediction was that all [Wall Street sell-side research] analysts would be more optimistic for firms that
were issuing debt and equity securities than for firms that were either not
engaging in such activity or were reducing their financing needs. An integral
part of the forecasting function that analysts perform is assessing the need
for external financing to fund operations. Thus, analysts are already
intimately aware of what financing needs a company is likely to have.
Accordingly, in periods leading up to a need to go to market for financing, we
predict that all analysts would tend to increase the level of optimism embedded
in their published research. This is exactly what we found."

"Even more interesting is that the optimism embedded in analysts' forecasts
differs depending on whether the firm is issuing debt or equity securities. For
firms that issue securities, their concern is the initial pricing of those
securities. The pricing of equity is largely determined by the long-term
performance of the firm, whereas the pricing of debt is limited on the upside,
with investors standing to recapture principal and receive interest payments.
Accordingly, we see that analysts' optimism varies with the type of security
being issued. For debt, the optimism is restricted to near-term earnings
forecasts (i.e., the next two years); for equity, the optimism is concentrated
in longer-term forecasts (i.e., growth forecasts, target price forecasts) and
stock recommendations."

In other words, not only is sell-side research impacted by the investment banking needs of the companies covered, but it's impacted in a very sophisticated and targeted way.  Interestingly, Bradshaw found that this was true regardless of whether or not the analyst's employer was current providing investment banking services to the client.  "The predictions in prior research were always that the affiliated analysts would be more optimistic than the unaffiliated analysts. This always seemed odd to me because unaffiliated analysts want to be affiliated so they can realize the benefits of associated investment
banking fees and other ancillary services such as mergers and acquisition work, private placements, asset financings, and so on."

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