Neither Greece teetering on the edge of bankruptcy nor the end of QE2 could keep the S&P 500 from rallying 4.1% over the previous four sessions. For optimists, these rallies may seem like a dream come true. For skeptics like me, they're opportunities to see whether companies trading near their 52-week highs have actually earned their current valuations.

Keep in mind that some companies deserve their lofty valuations. Visa (NYSE: V), MasterCard (NYSE: MA), and many other debit card issuers rallied dramatically this week following the decision by Congress to cap retailer swiping fees at $0.21, up from the original $0.12 it had discussed. For companies like Visa and MasterCard, this means significantly more profits than analysts had originally planned on.

Still, some companies might deserve a kick in the pants. Here's a look at three companies that could be worth selling.

Where's the beef?
Just because a company offers the latest life science technology capable of literally splicing and dicing your genes, that doesn't make it a compelling buy. Affymetrix (Nasdaq: AFFX) has shown promise on multiple occasions over the years, but its business leaves a lot to be desired.

Based on its reported results from 2007 and looking forward to next year, revenue has been on a steady decline and only now is it beginning to level off. A lack of urgency to innovate, and spending cuts from some of its customers have left the company struggling to return to profitability despite improvement in the company's gross margins. Its cash position is healthy, but shareholders would be wise to reconsider their positions given that Affymetrix has missed analyst projections often over the past few years. This may be a GeneChip off the old block, but I'd kick this stock to the curb if I were you.

High-speed exit
I almost smacked my head against the wall in disbelief when I noticed that Cogent Communications (Nasdaq: CCOI) had worked its way to a new 52-week high. The provider of high-speed Internet access and communications services to small and medium-sized businesses has often struggled to maintain profitability despite steady revenue growth. Now trading at 1,063 times trailing-12-month earnings, I have to wonder what traders are thinking.

Within the past six months, insiders have been cashing in on shareholder optimism, with 33 of the 38 insider transactions being sales. Also consider how murky Cogent's balance sheet is in relation to almost any of its competitors. Cogent's 240% debt-to-equity ratio and forward P/E of 46 are almost laughable next to AT&T (NYSE: T) and its 57% debt-to-equity ratio and forward P/E of 12. With countless cheaper alternatives available and insiders signaling that they agree, I can't see much reason to be a shareholder at this point.

Not every company from the dot-com bubble perished, but every once in a while investors need a wake-up call that this isn't the year 2000 anymore.

MicroStrategy (Nasdaq: MSTR), which soared to a split-adjusted all-time high of $3,130 back in 2000, is still a long way away from its glory days, but recent trading action in the stock would suggest the euphoria is returning. My advice is to keep an eye on its expenses and its margins.

Based on the company's recently filed first-quarter report, revenue rose by a solid 31%, but expenses jumped by a whopping 37%. Thus it's no surprise why MicroStrategy's trailing-12-month operating margins are decisively below 10%, while software big boys Oracle (Nasdaq: ORCL) and SAP have operating margins north of 30%. MicroStrategy does have a debt-free balance sheet and strong revenue growth in its favor, but if it can't control its expenses, it makes no sense to own when more efficient alternatives exist.

Foolish roundup
The companies highlighted this week are all expecting profits in the upcoming year with the potential for double-digit revenue growth, but they all share one other common factor: They're the expensive company in their sectors. Sometimes letting go of one company because cheaper and more efficient alternatives exist in the sector can be the way to go.

What's your opinion on these stocks? Are they sells or belles? Share your thoughts in the comments section below and consider adding Affymetrix, Cogent Communications, and MicroStrategy to your personal watchlist to keep up on the latest news in these stocks' respective sectors.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong. The Motley Fool owns shares or Oracle. Motley Fool newsletter services have recommended buying shares of Visa and AT&T. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy that never needs to be sold short.